Yes, you can offset the capital loss from one property against capital gain from another property.
It is not mandatory to sell the other property in the same year.
You can file ITR for FY 20-21 with capital loss. This capital loss shall be carried forward and would be available for set off against capital gains in future years.
Kindly ensure that the ITR is filed before due date to carry forward the loss.
We may assist you with filings and other compliance.