2020 distributions are not RMDs, so can be converted within 60 days. The one rollover limit does not apply.
Fidelity may be checking to see if your distribution was still within 60 days for the rollback or if you have already done one rollover in the last 12 months. Many people have already rolled back earlier distributions that were originally planned as RMDs. Vanguard is accepting them after you verify that you qualify. If Fidelity does not change their mind and you are getting close to 60 days, you can roll it over to another IRA that you may need to open.