Cause
You forgot to finalize the return for the previous cycle. However at some point you realised that the cycle date hadn't changed, so you changed it.
Solution
To fix this, the first thing to do is to change the GST Cycle Date back to the date of the end of the previous cycle (this is done in the GST panel of the Document preferences in the Edit menu).
Then, refer to “What do I do if I forgot to finalize my last GST report?”.