Is it possible that the university computer is running an outdated version of MATLAB? According to the documentation xline was introduced in 2018b. Type ver on the command line to see what version you are running. Otherwise it could be a path problem. Make sure that the directory where the xline.m file is found, e.g. C:\Program Files\MATLAB\R2020a\toolbox\matlab\specgraph\xline.m, is on the MATLAB path.