Mathematica is fiercely case sensitive. plot is unknown. Plot is a Mathematica function.
AppellF1 does a doubly infinite summation and for some arguments can be very slow to compute.
Plot may evaluate the function hundreds or even thousands of times when trying to show a precise image.
Sometimes for slow functions doing
for suitable numeric constants a,b,c can more quickly give you an approximate indication of the behavior of the function f[x].