A qualified bicycle reimbursement may be claimed across more than one month provided that the purchase was made in an eligible bike commuting month. You can claim reimbursement for the expense up to $20 times the number of eligible months in the calendar year. In this case, however, you would have had to have commuted by bike for three months in the tax year, to qualify for the entire $50.
Additionally, the remaining amount of the $50 qualified purchase made in TY 2020 may not be rolled over to TY 2021.