Hi @Gorilla_8 ,
Could you tell me the data type of these three fields: Título, ItemCost ,MasterID?
Please do not use patch to update a system field.
This kind of field will be generated data by system automatically, no need to update it.
You do not need to create new record with system field value, and you can not change system field value too.
System field is read-only.
Based on your description, I think maybe MasterID is a system field.
If so, I suggest you just update other fields.
Try this formula:
ForAll(ExpenseCollection; If(!IsBlank(LineItem);Patch(ExpenseDetails; Defaults(ExpenseDetails); {Título: LineItem; ItemCost: LineCost })))
Here's a doc about system field in sharepoint list for your reference:
https://sharepointmalarkey.wordpress.com/2008/08/21/sharepoint-internal-field-names/
Best regards,