Welcome to Community, @CraigBlack21.
I'll share the steps to fix the "taxReturnInvalidMTDEndDate" error. You'll want to select Grant Authority from the tax settings to do this. Let me walk you through how.
- Go to Gear.
- Choose Account and Settings.
- Tap the Advanced tab, and then the pencil icon in the Making Tax Digital section.
- Select Learn how to connect.
- Log in with your HMRC credentials.
- Click Grant Authority.
- Follow the steps that'll guide you through the rest.
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I'm attaching this link here to help you manage tax-related tasks. It has our general tax topics with articles: Manage taxes in QuickBooks Online.
With the above recommendation, you should be able to resolve the "taxReturnInvalidMTDEndDate" error.
Pleas don't hesitate to reach out to me anytime if you have other questions about tax refunds. I'll be around for you. Take care and have a great day ahead.