Hi,
When you open a report, MR uses S-1 for the Default Base Period. This is the system date minus 1, or December 31, 2019 for today. If that period does not exist in the company, you'll get a message like this. MR will then set the Default Base Period to CUR, which is the last open period.
If you are using the legacy provider, there must be at least one open period. If 2019 and 2020 were open years but had all the period marked as closed, you'd need to open at least one in order for MR to read the periods correctly.
Are you able to select 2019 from the Base Year drop down and 12 from the Base Period drop down, in the report definition? If you cannot, you'll want to verify that those periods exist in the company select in the report. If you are using the data mart, and those periods exist, you'll want to rebuild the data mart integration.