I am assuming you are NOT un-mouting "Vserver namespace root volume". before unmount, check attributes is_junction_active=true and junction_path is set.
also, pl. check the unmount job for further details:
POST :
https://<ip_address:port>/api/4.0/ontap/volumes/<key>/jobs/unmount
JOB:
https://<ip_address:port>/api/1.0/jobs/<key>
You may use below option during unmount
POST
https://<ip_address:port>/api/4.0/ontap/volumes/<key>/jobs/unmount
BODY
{
"force": true
}