I have already submitted my CIS for last month, which was accepted by the HMRC, however QB thinks this is still open and therefore won't let me submit for October! Help?

Thanks for reaching the Community space, @stephanie.s.

When you successfully file the CIS return via QBO, a prompt will show that it’s accepted by HMRC. The submitted status will show in your company and not in the agency’s portal.

It also goes the same way if you process the tax form directly through HMRC. The information will not flow into QBO and you'll only see the status on the tax agency's site.

Since it's already accepted by HMRC, let's open the CIS return and manually mark it as filed. I'll show you how:

1. From the Taxes menu select CIS.
2. Look for the CIS return that you need to change.
3. Click Prepare return.
4. Choose Mark as Filed from the drop-down beside Submit to HMRC.
5. Click Continue.

I'm adding these helpful articles to learn more about how the CIS feature works in the program: 

• CIS in QuickBooks
• Already using CIS in QuickBooks?
• Filing CIS in QuickBooks Online 

You can visit me here if you have any other questions. I'll be around to get you taken care of. Have a great day!