Hope you are better now. If you put it on a credit card or took out a loan you can deduct the full amount even though you pay it over time. If you are on a payment plan with the hospital or doctor, etc. then it's only the amount you pay. Either way you only can deduct the principal amount, not any interest.
You can only claim the amount you paid in 2020 on your 2020 tax return. You cannot claim amounts that are still unpaid.
The medical expense deduction has to meet a rather large threshold before it can affect your return. The amount of medical (including dental, vision, etc.) expenses that will count toward itemization is the amount that is OVER 7.5% of your adjusted gross income. You should only enter the amount that you paid in 2020—do not include any amounts that were covered by insurance or that are still outstanding. Of course, your medical expenses plus your other itemized deductions still have to exceed your standard deduction before you will see a difference in your tax due or refund.
To enter your medical expenses go to Federal>Deductions and Credits>Medical>Medical Expenses