If you try to execute something like ./matlab from your terminal you tell the shell to run an executable command "matlab" in the current directory. That would work if you were in the bin-subdirectory of your matlab-installation directory, or in a directory where you had a link matlab pointing to that executable.
Further, it seems that you've installed matlab with ~you/Documents as the installation-directory. That's not pretty. Matlab is not a document. On my machine I have all matlab-releases in a directory /usr/local/MATLAB/, and when I install matlab I do it as super-user so that it get properly installed with executables (links to /usr/local/MATLAB/R2020a/bin/matlab etc) in /usr/local/bin. That way I will have matlab on the shell-path so that I can start it from the shell like: matlab -nodesktop (my prefered way).
I suggest that you re-install matlab as super-user and select to create links in /usr/local/bin (that is the default suggestion IIRC). If you cannot do that and have no user with sys-admin privileges at hand I think you should still move the full installation from your Documents directory, and create a link to the matlab-executable in the bin-directory from a directory on the path you have in your shell (perhaps ~/bin).