You need to store the first four lines of that code in a file named tankfill.m, but the lines starting from the assignment to tspan should either go into a different file or should go on the command line.
In R2015a it was never valid to try to define a function at the command line, and it was never valid to try to define a function inside a script file (a script file is any .m file whose first executable line is not 'function' or 'classdef'). A later MATLAB release made it possible to define functions inside script files in some circumstances, but even in the very latest versions it is never valid to define a function at the command line.
