In Salesforce Lightning Experience, the Save button in a Joined Report may become grayed out (non-clickable). This occurs when one of the report blocks contains only a Record Count field and report details are hidden.
Steps to Reproduce:
The Save button in a Salesforce Joined Report becomes inactive when a report block has no visible columns displayed other than Record Count. Salesforce requires at least one field column to be visible in each report block before the report can be saved.
To resolve this issue:
000386760