I Just got hired in a brand new restaurant and 3 weeks later they say im too slow&stuttering, can they fire??

hey i have been doing my job great up to the last week when a manager cam up to me and put me on the spot and i got really nervice and frose up...then starting asking me questions about my job...i got 3 out of the 4 questions right...then tells me im not fit to work here,you stutter, your too slow,your not like the rest of everyone and i cant be having you on the floor": he gives me a menu and tells me to call him tomarrow..i call him the next day ..dont get him but get the isant manager and asked whats up...then he tells me we cant " grow with you" with our comany and were taking you off the work week...then i ask Does that mean im fired? and tells me no just that where nt puting you on....my question is can they legally do that? i spent 2 weeks training for this job and only now have they said somthing like this...also i was reading online and my freind is a law students and told me somthing about how this might go against the Americans with Disabilities Act of 1990???