@kpeters Looks like in the Source module "D11 Journal Manual Entries", you are trying to group "Text" formatted line item " Cust.Mkt ID". If Customer is one of the List in the model and MKT ID is a property or code of Customer List, you can use "FINDITEM()" to find the List Member in a Customer List formatted line item and refer that in SUM:
The reason for this error ("Data type mismatch: format other than LIST or DATE being referenced as a relation") , the line item using which you are trying to sum is a text formatted rather date or list.
In order to sum here:
1. Create another lineitem name it Cust.Mktlist and format it by Cust.Mkt list
2. Formula for Cust.Mktlist :- Finditem('Cust.Mkt',Cust.Mkt ID)
3. Now use this lineitem "Cust.Mkt list" in the sum formula.