TurboTax will take care of this automatically.

Maryland will exclude:

Up to $5,000 of military retirement income received by a qualifying individual during the tax year if the taxpayer has not yet attained the age of 65; or up to $10,000 of military retirement income received by a qualifying individual if the taxpayer is age 65 or over.

1. On the Where is this Distribution From screen, mark the radio button for Military Pension or Active duty military retirement [depending on the version of TT]. 
   
2. Enter the distribution amount (if it's not already there) and click Continue.
3. On the screen Was This Distribution from a Qualified Plan, mark the blue Non-Qualified Plan box.

TurboTax will calculate the exclusion on your MD return.

Where in Turbo Tax is the entry for retired military tax?  There was a button at the start of turbo Tax but it refereed to active duty and Guard only.

Go back to the personal information section. The initial question will ask if you are a member or former member, select yes. That will get you in the right places. Thank you all for your service.

@CaptDock

Where can I see that the $15,000 is deducted in military retired pay for 2019?  I do not see anything anywhere that shows this as line U of the Maryland form 502SU. 

If you intend to pay or have already paid, go to Tax Tools, Tools, Print Center and view your return. If you have not paid, you can pay now. Otherwise, you can:

1. Log into your return
2. Select state taxes
3. Enter the MD state
4. On the left, go to Tax Tools
5. Select Tools
6. Select View Tax Summary
7. You will see Summary of your MD Return at the top.
8. Locate the line that says Subtractions from Income
9. Click Back when done.

The only issue is you may have more things in the subtractions. You should have a number $15,000 or more.