I would suspect that both GE_Claim and GE_Reserve have a column named Month_of_file.  Prefix the column name with the appropriate table alias and you should be okay.

Clarification:

SqlDataAdapter cmdItemTypes = new SqlDataAdapter("Select ItemName,ItemModel,PurchaseDate,Location,DepartureDate,ItemType,AdditionalInfo From Items Where Location = " + location + "",...

Your parameter needs to be enclosed in quotes, or TSQL will interpret it as a column name, not a parameter value.