Do you have the `fpc` in your `PATH`? If not, you'll need to alter Geany's build command (*Tools → Set Build Commands* when looking at a Pascal file) to point to the full location of the `fpc` executable. Finally, you probably know it but you need to compile the program before running it, so if you were trying to *Execute* before *Build*ing, it won't work as the executable won't exist yet.