The error message suggests that there is a type mismatch between the database column type and the type of the corresponding property in the Customer class.
Since you have changed the id property in your Customer model to a string, you need to ensure that the corresponding column in the database is also of type varchar or char(36) to accommodate the UUID values.
If the column is already of type varchar or char(36), then you may need to modify the code that retrieves the customer record. Try updating the LINQ query to cast the UUID value to a string before comparing it with the customer number:
Customer customer = dbContext.Customers.FirstOrDefault(c => c.Id.ToString() == customerJson.CUSTNMBR);
This should retrieve the customer record based on the string representation of the UUID value in the "Id" property.
If the above solution does not work, then you may need to check your model class and database schema to ensure they are correctly configured to use UUID values for the Id property.