As far as I know, Paratext stores auxillary projects in a different folder to the main project. I wonder if your new computer has a copy of the standard project? If so, it should just be a case of picking it from the "Paratext project" dropdown list on the "Basic" tab.
If it can't find any other projects, then it's possible you have multiple paratext directories installed. This is particularly likely if you have paratextlite on linux - updates some months ago seem to have switched to use a directory under ~/snap/paratextlite/.... rather than in the home directory where PTXprint expects it to be. A symbolic link may help you in that case, or you can run ptxprint -p /path/to/ParatextProjects