I only need to enrol for master's thesis

If you only need to complete your master's thesis, you will need to (re-)enrol for the master programme ánd for the master's thesis. You must also re-submit the subject of your master's thesis in SisA Selfservice.

https://studhelp.uantwerpen.be/ro_helpdesk/knowledgebase.php?article=43

Information about the cost for re-enrolment can be found on

https://www.uantwerpen.be/en/education/admission-and-enrolment/enrolment/tuition-fees/

Also check out:

https://studhelp.uantwerpen.be/fsw/knowledgebase.php?article=167

Manual: I want to graduate in February. How can I request this? (uantwerpen.be)