One of the classic issues in Linux: the system can't see the command the user is trying to execute. That is either because the command doesn't exist (mistyped the name or actually never compiled/installed) or because it is somewhere where the system is not looking. Can't tell from you description which one is the case.
If the program exists, it needs to be placed in the PATH, or the whole program location needs to be typed (something like /directory/that/contains/psipred). If you don't know what this means, let me know and I can explain further.