Hey @Kadir_Gokduman, welcome to the Nextcloud community and thanks for supporting Nextcloud snap 
Sorry to hear you are having issues with your Nextcloud snap installation. If you’ve come this far, you’re on the right track! There is a good chance someone in the community has had a similar issue and knows how to resolve it.
Below are some links to how-to’s answering frequently asked questions and may help you help yourself;
- How to install Nextcloud snap
- How to configure Nextcloud snap
- How to edit config.php for Nextcloud snap
- How to use OCC management console for Nextcloud snap
- How to revert Nextcloud snap after failed update
- How to manage failed automatic update in Nextcloud snap
- Nextcloud snap wiki and support
Pay special attention to the docs pertaining to mounting external media and permissions / privileges:
What permissions should external media have?
See FAQ’s and Snap confinement
Removable media or external storage must be mounted to either /media or /mnt as root with root permissions and connected to Snap!
The interface providing the ability to access removable media is not automatically connected upon install, so if you’d like to use external storage (or otherwise use a device in /media or /mnt for data), you need to give the snap permission to access removable media by connecting that interface:
sudo snap connect nextcloud:removable-media