The following information will help in case you run into some issues while deleting Project Service Automation solutions. Usually, the correct order for deleting the Project Service Automation solutions from organization is the following:
- ProjectService_Trail (if exists)
The solution may fail to update due to customizations made on the system. If you install solution A, then install Solution B which has a dependency on Solution A, you will not be able to uninstall Solution A. The same happens if you install solution A and then make unmanaged customizations that reference components from solution A. You may no longer be able to uninstall solution A. In this case you will not be able to delete the base PSA solution ProjectService.
If you have added any related customizations, a message will appear which can help you access the dependencies you will need to remove.