You have toggled Multiple Selection Mode in Fiori > Service > Unassociated Emails but you cannot tick more than one email.
Reproducing the Issue
1. Go to the Service workcenter.
2. Go to the Unassociated Emails view.
3. Click Toggle Multi Select.
4. Select several emails.
Result: when you try to select several emails this is not working.
This is a known issue.
This functionality is currently still under development and will be available in a future release.
KBA , SRD-CC-FED , Feeds , Product Enhancement