Online accounts cannot be deleted. You can remove the tax return information.
If you have not paid for the online edition you are using, have not filed your tax return or registered the Free edition, then you can clear your return. Click on Tax Tools on the left side of the program screen while working on the 2020 online tax return. Click on Clear & Start Over.
You can not delete or merge accounts.
You can try to clear all of the information in your TurboTax account for this year but you will need to Contact Customer Support to have an account deactivated for more of a resolution.