It's because your MATLAB installation directory is not added in your $PATH. Assuming the MATLAB installation directory is /usr/local/MATLAB/R2019b, you need to add the sub directory "bin".
If you have sudo privilege, create a symbolic link in /usr/local/bin.
sudo ln -s /usr/local/MATLAB/R2019b/bin/matlab /usr/local/bin/matlab
If you don't have sudo privilege, change your PATH environment dynamically.
It might be convinient to add path in your bashrc file.
and add the export PATH command written above in bashrc.
Then, save the bashrc and reload.