I assume you are following the introduction course. (if not, then following answers doesn't apply)
Two guesses what might cause the errors;
1. The modeler version used for the course content (and thus the files supplied) is 6.6. Opening these modules or project packages in 7.6 will cause errors.
2. Possibly you have set your navigation items and connected them to pages as target; by deleting your old work, the target pages are gone and will cause errors.