Given that a user never defines what type of variable something is in the import definition, this error message is impressive
"The given value of type String from the data source cannot be converted to type bigint of the specified target column."
Clearly the system makes assumptions about datatype, yet then I as a user should be able to modify it. So either everything is text, or I should be able to edit datatype.
SEcondly, there is a problem with the message itself as it doesn't specify which variable this error is related to, making troubleshooting close to impossible.
Attached file shows example.