Matlab with printing error

Hello! So I have a script. Until now it worked good, but I changed some internal things and now a print won't work. It is not called in the code too often, yet I don't know what it has. The script starts with:

fid = fopen('Output_Results.txt', 'wt');

and the error is on the first line:

fprintf(fid, 'Frame path: %s\n', frame_path);

fprintf(fid, 'Core path: %s\n', core_path);

fprintf('Frame path: %s\n', frame_path);

fprintf('Core path: %s\n', core_path);

idxCore = idxCore + 1;

error is:

Error using fprintf

Invalid file identifier. Use fopen to generate a valid file identifier.

Error in compare_bun (line 135)

fprintf(fid, 'Frame path: %s\n', frame_path);

I use Matlab R2020b, TargetLink 5.1p1

[KSSV]

Check the value of fid..... the error is clear, the required file is not opened. Did you change the path after openeing the file to write?

Also what are the values of frame_path. etc..? Show us the complete code.