Whilst I was importing my last calendar into my main Google calendar I received this error despite being the owner of both calendars!
A quick search showed that I’m not the only one to have faced this issue. The resolution that worked for me was to open your exported .ics calendar file in a text editor such as Notepad (best to take a back up copy first) and select Edit > Replace
In the Find what: field select UID: and in the Replace with: field type UID:X (basically it can be X or replace X with any alpha-numeric character (a-z,0-9)
Save the .ics file and redo the calendar import. Now it should import without error.