With the limitation information given, here is my idea:
Pyschotoolbox is a "3rd party product". In my experience, 50% of all problems relate compatibilty issues. Please verify if the Pyschotoolbox version you attempt to use is compatibily with the underying MathWorks products.
The error message suggest this command is either not existing at all or cannot be found (== not on the path). I suggest you look if you have a screen.m (or mlx) file on your computer and go from there.
