Good morning John.
When dealing with your GST return, you must be in the fiscal year you are generating the return for.
So, start by going to Setup > Fiscal Years. In the top-right corner of the screen, change the fiscal year to the prior year, the one you are wanting to generate your GST return for. Close up the fiscal years screen, and attempt to generate your GST return for the end of that fiscal year. This should work for you, and it will save in the correct year. Once done, you can change your fiscal year back to the current year, and continue working as normal.
I hope this helps!
