Two things to try...but use both in succession.
Close TTX, then
1) rt-click on the 2021 TTX startup icon and select "Run as Administrator"
Once started:
2) go to the top menu, select the "Online" popup/pulldown menu and select "Check for Updates", and run any updates.
Then try to open that file.
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Otherwise, time to call Customer Support:
What is the TurboTax phone number?
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Remember too, you can only print and file by mail at this late date.