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Hello there, @aandklawncarect.
Good to see you here in QuickBooks Community. Allow me to help and provide some insights about Connecticut (CT) registration format number in QuickBooks Online (QBO).
The format for CT DOR Tax Registration Number should be 10 or 11 digits and the last three are always 000. I suggest calling your State Agency to make sure you have the correct CT registration number.
Here's how you can update the correct format in QBO:
If you need further assistance with the steps, you may contact our QuickBooks Online Support Team. They have additional tools to pull up your account in a secure environment and do a remote session.
Please let me know how it goes by clicking the Reply button below. I'll be here to keep helping. Have a good one!