theta0 = tand(v0y/v0x)
while theta0 > 90 || theta0 < 0
% where the '>' gives an error message
Changing this to a while loop should fix your issue. A for loop counts for a specified number of iterations, the while loop iterates while some condition is met. So for your case you want this loop to run while theta0 is greater than 90 or less than 0. when that condition is no longer true, then the loop ends. Need to make sure that the code inside the loop changes theta0 or it will loop forever. If your do not plan on changing theta0 inside the loop, then Paul is correct above with using an if statement.