Challenge Not yet complete... here's what's wrong:
There was an unexpected error in your org which is preventing this assessment check from completing: System.DmlException: Insert failed. First exception on row 0; first error: FIELD_INTEGRITY_EXCEPTION, User: id value of incorrect type: 00536000002OPa4AAG: [User__c]

I saw a post reply saying to delete USER and recreate the field but I am unsure if I am supposed to create a new OBJECT under Campsite Reservation (directions say "Add a custom field...")

This challenge has given me fits for two days.  This is my latest error message I managed to fix the two previous...  THANKS. 

I am getting this error in the Data Modelling - creating object relationships challenge:
There was an unexpected error in your org which is preventing this assessment check from completing: System.DmlException: Insert failed. First exception on row 0; first error: FIELD_INTEGRITY_EXCEPTION, User: id value of incorrect type: 005i0000001YPgCAAW: [User__c]