The most common cause of this error is a Project Activity is missing a Type.
Project Activities MUST have a Type, of either Task or Appointment.
Saving a Project record that contains a Project Activity with no Type will cause Errors. If you try assigning a Type to the Project Activity (and that Activity had at some point been saved without a Type) this can still cause an Error when Saving the Project.
We recommend deleting the Project Activity entirely from the Project, and recreating it. Also, check to see if the Project Template the Activity was created from, may have a Blank Type value.
In the example below, the Project Template itself has a Project Activity that does not have a Type assigned. You will need to Edit the Project Template and make sure a Type is selected for every Project Activity.
Review ALL your company's Project Templates to make insure all Project Activities in every Template have been assigned a Type.