Problems with Nextloud App

We are Ageny for Videoproduction. We use Nextloud for our backups. Inside our Workstations is on HD only for Nextcloud synchronization. - We synced the main files of our Nextcloud on the HD - now we want do delete some of the files from the HD, therefore we selected all the files, we dont need and try to start the the snycronisation process but nothing happens. It tells us every time more space is needed on the local harddrive.

Is the solution to delete the data on the Harddrive? Does the app not work like the dropbox app, where i pick my needed data an the other data will erased from my lokal drive ? - What will happend if i delete the data manual? Will the data also be erased on the nextcloud server?

Thank you in advcance for your help!