while sending mail
Error message is clear that User Setup is not existing for user who is trying to send the email.
please go to User Setup and add the proper setup
In user setup that user exist......
It seems it is expecting '' (blank) user in the user setup.
Something wrong with code or some other setup.
you have to debug it or contact partner.
It is most likely caused by a blank 'salesperson/purchaser code" or "approver id" in the approval user setup for the particular user requesting approval. Also ensure that all emails for all users are entered correctly in the email columns along with the appropriate approval limit settings in the approval user setup.
Run query 1511 "User IDs by Notification Type" and check the result for column recipient_user_id (NAV 2016/2017). Look out for blank rows in the recipient_user_id column.