A report can't be shared if the Venue Group used in the report has not been shared with all users.
To fix this click on "Groups" at the top of the page.
Find the group that is used in your report.
Click on "More" next to the group
Tick the box to "share with all users".
Click "Save Group"
Now click "Load" to load your saved report.
Find your saved report and click "More" and then "Edit"
Choose who you would like to share your report with.
Click "Save Report"
This will now share the report with those users along with the venue group that you have created for the report.