This sounds like there was some kind of caching issue. Can you try closing all the sessions associated with that browser? Reopen the browser and try logging back into Pega 7 and them attempting to do the save as again?
I realize there are several people with similar errors. For me to better support you, I would need more information such as, which course, lesson and exercise and specifically which steps in the exercise were taken prior to the error message occurring.
Usually error messages are caused by previous actions or settings that have been made. Without more information prior to the error I am unable to replicate it in a test environment and provide you a solution.
I recently faced the same error message on a different type of rule, specifically a REST Service rule. In the end, I managed to solve it after discovering the AccessGroup specified in the Service Package rule and linked to the REST Service Rule didn't exist anymore. I changed it to a valid value and I was able to do the 'Save As'.