I want to mark my monthly March VAT return as submitted because I submitted it manually

It's nice to see you here in the Community, @altonfarms. 

I'm here to guide you on how to mark your March VAT return as filed. Here's how:

1. Go to Taxes from the left menu.
2. Select VAT at the top of the screen. 
3. On the Open Returns section, click Submit return.  
4. Make sure the Accounting period, End and Submission dates are correct. 
5. On the Submit to HMRC drop-down, click Mark as Filed.
6. Click Continue. 

Once done, the system will create a journal moving the amount of VAT due from the VAT Control to the VAT Suspense account. You can also close your books to protect your transactions in earlier accounting periods from being changed.

For more details, you can check out this helpful article: Submit a VAT return in QuickBooks. 

This will get you back in business in no time, @altonfarms. 

If you need further assistance, please let me know. I'll be around to help.