And what is the relationship you have defined between Customer and Sessions?
It seems you would need three tables with a "join" table that connects a given Customer record to a specific session record, unless your sessions table is the join table and there's a "classes" or "subjects" table that you didn't tell us about?
With the right relationship, this calculation can produce the comma delimitted list you want and it would indeed be defined in the Customer table:
Substitute ( List ( Sessions::SessionName ) ; ¶ ; ", " )
But it won't work without the correct relationship.