I am getting this error " BadRequest. The request failed. Error code: 'InvalidApprovalCreateRequestAssignedToNoValidUsers'. Error Message: 'Required field 'assignedTo' contained no valid users in the organization'."

When I am using direct value in Assigned to, the flow is working, when I am adding lookup column Endorser which is lookup datatype column which uses USER table to get data.
How can I get this fix?
I need to use that column.