Hello!
Sorry you are encountering this frustration! The error here either means there is an issue in the program or there might be a problem with the plan itself. I would suggest reaching out to support so we can make sure to fix the problem.
As for "Make a Copy", here is an explanation about the current design intention:
The difference between "make a copy" and legacy "save as" is all about which plan is open and which plan actually saves the changes.
Say I have "original plan" with unsaved changes and I make a copy of "original plan" and call it "changed plan".
Which plan stays open? "Original plan", with its unsaved changes exactly how it was.
Which plan actually saves the changes? "Changed plan", which will be closed and not open.
Legacy save as worked differently. Say I have "original plan" with unsaved changes and I do a legacy save as of "original plan" and call it "changed plan".
Which plan stays open? "Changed plan" does.
Which plan actually saves the changes? "Changed plan", which will be open. "Original plan" will be closed.
We have stepped away from legacy save as because of the confusion it causes in regards to linked layouts. Say you have "original plan" that is linked to a layout with live views. Doing a legacy save as of "original plan" would result in "changed plan" being open and "original plan" being closed. The confusion comes from the fact that the layout is still linked to the "original plan", which is now closed. Additional edits to the open plan ("changed plan" in this case) will not be reflected in the layout.