How to solve the "Cannot load default column mappings. Default user not found“ error message when us

Navigate to the Site Manager -> Settings -> System section and look for the "Default user ID“ setting, usually this is set to number 53 (global administator).  The corresponding user is used when inserting data via Import toolkit, you need to ensure this property is filled by the user ID of the user who exist in the target DB (where you import data to). Import Toolkit