fconvg = int(k*j, x, 0, t)
This line should probably come after the if-end.
The reason that you get
is because you are not assigning the function output to any variable. You need to call your function like this:
fout = AHMconvolve(...
^^^^^^ you need to assign to an output like this!
Yes I deliberately chose a different name to what it is named inside the function, to emphasisze that you are worknig in a different workspace, and the variable names used inside the function are (and should be) totally irrelevant.
How to call functions with output arguments and assign their outputs to variables is explained in the introductory tutorials: