Deducting Medicare Part B premiums as a self employed business expense

Medicare plan B payments are qualified as Self-employed medical insurance premiums and should be entered under Business instead of in the SSA-1099 Social Security Benefits section.

Go back and delete it from the SSA-1099 entry screen. Try going to the search box by My Account and type in Social Security Benefits. That will give you a Jump To link to take you directly to it.

OR
Enter a SSA-1099, SSA-1099-SM or RRB-1099 under
Federal Taxes tab
Wages and Income tab
Choose Explore on my own or I'll choose what to work on (if it comes up)

Then scroll down to Retirement Plans and Social Security
Social Security (SSA-1099. RRB-1099) - click the Start or Update button

Then under Business Expenses, Click Start or Update by Other common business expenses
Then click Update by Insurance Payments
Then click Update by Health Insurance Premiums

Self-employed health insurance deduction goes on Form 1040 Schedule 1 line 16 (then to 1040 8a), as long as the expense is not greater than your net self-employment income. If it does exceed your net self-employment income it gets split automatically. An amount equal to your net self-employment income goes on Form 1040 and the remainder gets added in to medical expenses on Schedule A.