Now it's giving me a red box saying that 201905 is outstanding.
Hi Bexter12 - this could be because you may have answered YES to the question:

Do you want the system to create a return for the next month?
If you select the option to create a return, the return will be added to the current range of declarations.
Please select 'Yes' if the return needs to be generated or 'No' if you do not want the return to be automatically generated.
You can leave that red until you need to pay it at the end of May/beginning June. It should not interfere with your April submission.


or it could just be me!
Nope - definitely not you. The system is not user friendly at all.