Bash + bsd-games package, 28

• 18 bytes saved thanks to @Dennis.

primes 1|egrep -wm1 [$2]{$1}

Input given at the command-line as n followed by k as a non-delimited list of digits.

Try it online.

JavaScript (ES7), 100 bytes

Takes input as number of digits n and string of allowed digits s in currying syntax (n)(s). Returns undefined if no solution is found.

Works rather quickly for up to 6 digits, might work for 7 and definitely too slow -- and memory hungry -- beyond that.

n=>s=>(a=[...Array(10**n).keys()]).find(i=>eval(`/[${s}]{${n}}/`).test(i)&&a.every(j=>j<2|j==i|i%j))

Test

let f =

n=>s=>(a=[...Array(10**n).keys()]).find(i=>eval(`/[${s}]{${n}}/`).test(i)&&a.every(j=>j<2|j==i|i%j))
                                        
console.log(f(5)("247")) // 22247

Pyke, 18 16 bytes

j;~p#`ljqi`Q-!)h

Try it here!

Runs forever if no values found

Jelly, 12 bytes

DL×ÆP
ṗḌÇÐṀṂ

Takes a set and an integer as command-line arguments. Prints 0 if no solution exists.

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How it works

ṗḌÇÐṀṂ  Main link. Left argument: A (digit set/array). Right argument: n (integer)

ṗ       Cartesian power; yield all arrays of length n that consist only of elements
        of the array A.
 Ḍ      Undecimal; convert all generated digit arrays to integers.
  ÇÐṀ   Keep only elements for which the helper link returns a maximal result.
     Ṃ  Take the minimum.


DL×ÆP   Helper link. Argument: k (integer)

D       Decimal; convert k into the array of its base 10 digits.
 L      Take the length.
   ÆP   Test if k is a prime number. Yields 1 or 0.
  ×     Multiply the length and the Boolean.

Mathematica, 64 bytes

FirstCase[Tuples@##,x:{f_,___}/;f>0&&PrimeQ[y=FromDigits@x]:>y]&

Pure function where the first argument is the (sorted) list of allowed digits and the second argument is the allowed length. Tuples@## computes all lists of the allowed digits of the allowed length, then we find the FirstCase which matches x:{f_,___} such that the first digit f is not 0 and the integer y=FromDigits@x is prime and replaces it with y.

JavaScript (ES6), 86 bytes

Takes input via the currying syntax, e.g., (4)('12')

n=>(d,F=(i,P=j=>i%--j?P(j):1==j)=>P(i)&&`${i}`.match(`^[${d}]{${n}}$`)?i:F(i+1))=>F(2)

'use strict';

const G=n=>(d,F=(i,P=j=>i%--j?P(j):1==j)=>P(i)&&`${i}`.match(`^[${d}]{${n}}$`)?i:F(i+1))=>F(2)

const submit = () => {
  console.clear();
  console.log(G(+n.value)(d.value));
}

button.onclick = submit;
submit();

<input id="n" type="number" min="1" value="4" />
<input id="d" type="text" value="12" />
<button id="button">Submit</button>

To be run in strict mode (for tail call optimisation [TCO]). If your environment doesn't support TCO it will result in a stack overflow error for primes larger than the environments stack.

For invalid inputs it will run forever.

Note:

• Chrome (>= 51) users can go to chrome://flags/#enable-javascript-harmony and enable this flag to run the above snippet with TCO support.
• Safari (>= 10) supports TCO

Python 2, 66 bytes

f=lambda n,s,k=1,p=1:10**~-n<p%k*k<s>=set(`k`)or-~f(n,s,k+1,p*k*k)

Try it online!

Takes input like f(3,{'9','3','8'}).

Python has no built-ins for primes, so the function generates them using Wilson's Theorem to check each potential value for k in turn for being prime.

The chained inequality 10**~-n<p%k*k<s>=set(`k`) combines three conditions on k:

• 10**~-n<k: k contains at least n digits. We don't need to check exactly since if we reach more digits, there must have been no solution
• p%k>0: k is prime, via the Wilson's Theorem condition with p=(n-1)!^2. Since p%k is 0 or 1, this can be combined with the previous condition as 10**~-n<p%k*k
• s>=set(`k`): All digits in k are in the set s. This can be spliced in because Python 2 considers sets as bigger than numbers.

If the current k doesn't satisfy all of these, the function recurses onto k+1, adding 1 to the resulting output. Since the output terminates with True which equals 1, and k starts at 1, the output is k. This parallel tracking of k beats outputting k directly on success.

Perl 6, 43 bytes

->\n,@k {first *.is-prime&/^@k**{n}$/,^∞}

Runs forever if no solution exists.

MATL, 17 bytes

wlwX"1GXNUStZp)l)

This function accepts two inputs, an integer specifying the number of digits and a character array indicating the possible values. In the case of no primes, an error is shown.

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Explanation

        % Implicitly grab two inputs. First as an integer (N), second as a string (OPTS)
w       % Reverse the order of the inputs
l       % Push the literal 1 to the stack
w       % Pull N back to the top of the stack
X"      % Repeat OPTS N times 
1G      % Explicitly grab N again
XN      % Get all N-character combinations of the repeated version of OPTS
U       % Convert each row from a string to a number
S       % Sort them in ascending order
tZp)    % Grab only those that are primes
l)      % Retrieve the first prime
        % Implicitly print the result

Pyth - 13 12 bytes

f&P_T!-Tz^Tt

Test Suite.

Sage, 62 bytes

lambda l,d:[p for p in primes(10^(l-1),10^l)if set(`p`)<=d][0]

Takes input of the form: f( 4 , {'1','2'} )

Brachylog, 15 bytes

tL∧?h~lṗ.dẹp⊆L∧

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This is fairly slow.

Explanation

tL                Input = [H, L]
  ∧
   ?h~l .         The Output is a variable of length H
       ṗ.         The Output is a prime number
          ẹ       The Output's digits...
        .d        ...when removing duplicate digits...
           p      ...is a permutation...
            ⊆L    ...of an ordered subset of L
              ∧

Perl5, 77 bytes

($n,$d)=@ARGV;/^[$d]{$n}$/&&("x"x$_)!~/^(..+?)\1+$/&&print&&die for 2..10**$n

Run like this:

perl -le '($n,$d)=@ARGV;/^[$d]{$n}$/&&("x"x$_)!~/^(..+?)\1+$/&&print&&die for 2..10**$n' 4 12

Ruby, 77 76 bytes

->n,l{(10**~-n..10**n).find{|n|(2...n).none?{|x|n%x<1}&&!n.to_s[/[^#{l}]/]}}

Input format: a number and a string.

Example:

->n,l{...see above...} [6,"555555555515555555555"]
=> 115151