C++ (gcc), 124 113 bytes

-11 bytes thanks to @ceilingcat

#import<string>
auto a(int n){std::string f;for(int i=30,j,k;++i<127;f+=i*!j,j=n)for(k=i;k;k/=2)j-=k&1;return f;}

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Iterates through the printable character codes, finds those that match the number of 1 bits desired, returns the string containing those characters concatenated.