Without asking, I can see your problem.
You’re using the wrong admin for NethServer/NextCloud!
On first setup, you need to login to Nextcloud with User admin, PW: Nethesis,1234
Then, activate LDAP. I advise using a group called nextcloud-users, and only allow users of that group - otherwise all system users will also be visible in Nextcloud…
Then, under Users, give admin permission to the NethServer Admin (And maybe other users).
After that, don’t forget to change the NextCloud admin Users Password (And write that down!), even though you’ll most likely never need to use that user again, as all users with admin permissions in Nextcloud are enough.
Then go - as admin - and install and activate “External Storage” under Apps.
Configure “External Storage” under Settings (Einstellungen).
Note: There are “Admin” configured External Storage and Private ones, each has to be activated by the admin. The Private storage is meant for eg the Users home directory.
The rest are typical “Shares” like on any Windows…
See these Screenshots from my home server (NethServer!):
This is what it looks like for the user (under Files in NextCloud):
I usually limit the External Storage for users to SMB… (Only the home directory is needed…).
This instruction should get you started!
My 2 cents