Your command is not correct. You are trying to start telemac2d.py using a relative path, from an example folder. Therefore, the shell tries to locate telemac2d.py script in the example folder: this can't work at all.
The correct syntax is:
telemac2d.py t2d_bump_FE.cas
However, in order for this command to work, Telemac Python scripts directory should be first put in your PATH environment variable:
set path=C:\opentelemac-mascaret\v8p3r1\scripts\python3;%path%