When trying to move an e-mail from Interactions in Tickets to an existing Ticket, no Tickets are available for selection.
You are using Employee Support scenerio and Ticket work center view is not assigned to you.
Reproducing the Issue
Go to Service work center.
Go to Employee Support view.
Open Ticket 123(where 123 is the Ticket ID).
Navigate to Interactions facet.
Select the e-mail you want to move to an existing Ticket.
Click on the cogwheel icon and select 'Move to Existing Ticket'.
When you click on the value help, no Tickets are displayed for selection.
This is because Tickets work center view is not assigned to the user.
Even if Employee Support is only used, Ticket Work center view assignment is required for the Ticket value help to show any instances.
The Ticket list will show all the Tickets including Employee Support List.
In case you do not want to work with 2 lists, you can assign only Ticket list which has all the features available in Employee Support list and also Ticket value help would work.
This is known restriction due to access context. Ticketvalue help is tied to Ticket list for access context and only works if the Ticket work center view is assigned to the user.
This is the expected system behavior.
Interaction, Employee Support , KBA , move to existing ticket , LOD-CRM-SRP , Service Request Processing , Problem