This usually happens if one of the block was added to display the record count (without any column added) and can be replicated by following the steps below:
1. Go to Reports tab
2. Click "New Report..."
3. Select a report type then click "Create"
4. Change the format of the report to "Joined Format"
5. Select a numeric field from the left panel then click and drag next to the existing block to create a new report block with the same report type or click "Add Report Type" to add a different report type. (If a different report type was added, remove the existing columns.)
6. Hide the details of the report by clicking "Show" then unticking the "Details" option.
7. Remove any summarized field on the second report block aside from "Record Count".
The possible reason is because one of the report blocks has Record Count as the summarized value but no columns added as a detail.
To resolve the issue, click "Show" and select "Details" to identify which block has no column then drag and drop a field from the list of available fields on the left panel to the report block. The Save button should not be greyed out anymore