How to sync a folder in the server with a folder in the client?

Nextcloud version: 21.0.7
Operating system and version: Ubuntu 20.04

Hello. I am quite noob in anything linux or even CS related. Around a year ago I installed a docker version of nextcloud, and somehow managed to point user files to the outside of the container, in /srv/nextcloud. I don’t remember how I did that. There I have index.html, nextcloud.log and directories for my users (user1/ and user2/ ).

I also installed Calibre-Web to my server. It’s a eBook library management interface.

I synced my eBooks library with the server using nextcloud.
And in Calibre-Web, pointed the library in /srv/nextcloud/user1/files/Books.

Now, calibre-web works fine, I can use my library, view and download books using its web-interface.

The problem is, when I make changes in the library, those changes are not synced with my local folder. Therefore gets reverted back when Nextcloud syncs my local folder.

I understand I should not modify the server files directly. So I thought creating a local external storage folder might work. I tried to create one following the manual.

First I created a folder /srv/books. made its user to www-data, same as my srv/nextcloud folder. gave its permissions (0750) .

Folder name: Books  
External Storage: Local  
Authentication: None  
Configuration: /srv/books  
Available for: user1  

But there is a red warning sign to the left of the line, and when i click the confirmation button it just reemphasize the red warning sign.

I tried to check the folder using nextcloud web interface. There is a folder named Books but when I click it says:

“External Mount Error
There was an error with message: Empty response from the server. Do you want to review mount point config in admin settings page?”

And clicking yes sends me back to the External Storages config page.

Can anyone help me?