"...I honestly don't know what caused this with chebfun"
You modified the function (perhaps accidentally) by adding some code outside of the function definition. By doing so you turn the Mfile into a script (which of course cannot be called like a function with input arguments).
"I don't know how to fix this..."
Remove any code that you added outside of the function definition. Users sometime do this accidentally at the very start of a function help when opening in the editor or when copy-pasting a function from somwhere, e.g.:
ooops % <- this turns the Mfile into a script!
% My wonderful function cannot be called :(
function myfun(inp)
..
end
If you are unable to find and remove your extra code then just download that function again.
