I'm getting a message "You've maxed out the federal $10,000 deduction for itemized state and local taxes" even though my taxes entered are only $4,600. Why?

There are other items in that.  SALT is State And Local Tax.  Which includes property tax, any state tax paid like for last year’s return and includes any state withholding from your W2s and any 1099s you have.  You can only deduct up to 10,000 (5,000 MFS) for SALT State and Local Taxes.