Select the form with the red exclamation mark by clicking on it and click on the "Errors icon" at the top of your forms and see if it shows you an error message. The error should show in a box below the form.
You can also try running Smart Check to see if it shows you the error.
If you are using TurboTax Desktop or TurboTax Online you can run Smart Check by entering "smart check" in the search bar and clicking on the "jump to smart check" link.
It should look something like this:
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