We don't have enough information yet to give you a definitive answer. Remember that we cannot see your tax return, nor the screen you are on.
dmertz is quite correct that TurboTax will prorate the annual contribution limit, except when you have a mix of Self and Family coverage and when you have the Self-only HDHP coverage on December 1, 2018 (I assume that dmertz was not presented with the exact description of your situation).
If you have a mix of Family and Self coverage, TurboTax will take the Self limit and add to it a prorated amount for the month(s) you had Family coverage. When I enter data similar to what you have said, I get a limit greater than the Self-only limit, even with only one month of Family coverage.
Perhaps you mean that you get the Self-only limit for the months you had the Self-only policy and a prorated Family limit for the months you had Family coverage, as opposed to getting the full annual Self-only limit for all 12 months and then adding the Family prorated limit to it? You can't get the last-month rule AND additional limits for some months with Family coverage.
If this does not work for you,
then please make a new post in which you indicate:
- your HDHP coverage and for how many months
- your spouse's HDHP coverage and for how many months
- your HSA contributions (both through your employer and directly to the HSA)
- your spouse's HSA contributions (both through your spouse's employer and directly to the HSA)
- the amount of the excess
- whether or not either of you went on Medicare and what month
- whether or not (and the amount) of carryover of excess contributions from 2017 for either of you
