To resolve this issue you will want to index at least one of the columns in the Email Tickets list. Since the link between the Ticket and the Email Tickets list is the Related Ticket ID column, this is the column you should index. To do so, an Admin user will need to perform the following steps:

1.       Go to the Site Contents page:

2.       Click on the ellipsis in the Email Tickets list to go to the Settings page:

3.       Once on the settings page, scroll down below your list of column and click “Indexed Columns”:

4.       Click “Create a new index”

5.       Choose “Related Ticket ID” as the Primary column. There is no need to set a Secondary column:

Click Create to set up the Indexed column.