If the expense and reimbursement both happened in 2021, then yes.
You can either subtract the reimbursement from the expenses you originally paid and enter the new expense amount, or enter it after you've finished entering all of your medical expenses. Either way, the reimbursement simply reduces your medical expense deduction by that amount.
If you got a 2021 reimbursement for medical expenses you paid in 2020 or a prior year, include the reimbursement or the amount of the medical deduction you got in the prior year (whichever is less) on your 2021 taxes.
Example: Last year, Jo had unusually large medical bills and could take the itemized deduction. They got a $1,200 medical expense deduction as a result. In 2021, they got reimbursed $800 from their insurance company. Jo would report the lesser amount ($800) as a miscellaneous other income item.
If you weren't able to get a deduction for the medical expense (usually because you took the standard deduction in that prior year), then don't mention the reimbursement on this year's return.
For more information, see Where do I enter my medical expenses?
