Looked for a while on the internet and spent time not finding the solution. So here it is:
- At your Synology NAS, just go to Control Panel -> Group
- Select user sc-syncthing and click ‘Edit’ button at the top
- Under 'Permissions’tab, add Read/Write access to share folder you would like Syncthing to have access to.
- Click save and you are done!
Good luck