is this your working directory?
Usuallly, there is at least one PATCH1 folder created after mt_prep_snap. Try to compute everything there.
Here it is:
StaMPS relies on the structure which is created after running mt_prep_snap. Wherever this command is executed (ideally an empty folder, don’t mix it with the exported files from SNAP [dem, diff0, geo, rslc]) should be your working directory until the end.
I also see that you are using StaMPS 3.3. There is a new version (4.1 available with several fixes): https://github.com/dbekaert/StaMPS