spell incantations and speech impediments

[Lotion]

Would somebody with a severe speech impediment be unable to cast a spell that requires them to pronounce a word they are unable to?

[xonn]

Without specifics, I am going to say no.

A speech impediment which is such that other PCs might be able to understand them, would be okay to cast with.
If you are thinking of making a concept where your speech impediment is so horrible that no living being can understand you, then you're better off not making them a magicker.

This is an RP mud after all. So RP according to the specifications and limitations you set for yourself in your own application.