On your 1099-INT, lines 15, 16 and 17 refer to how much tax is withheld to apply against your state income tax. Texas doesn't have an income tax, so all three boxes should be blank.
It does happen that sometimes the software that prints the 1099-INTs automatically puts in the state code whether or not it is needed. In this case, just don't enter the state code in box 15 when you enter your 1099-INT, and blank it out if it is already there.
This is an exception to our general mantra of "enter everything you see".
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