If you change your system to be a mixture of PureOS and other sources, then your system is no longer a PureOS system.

Possibly you can revert your system to again be a PureOS system by removing those other sources in Apt and removing all packages you installed from those other sources, but since we have no control over what those npn-PureOS packages did on your system while installed we can only guess, and the only way we can help you ensure that you truly run only PureOS is to tell you to reinstall from scratch and never install non-PureOS code on your system.

I used the nextcloud repository because the application was not present in those of pureos.

So I can rephrase my request: why the nextcloud client is not present in PureOS repositories ?

Thank you for clarifying the nature of this issue. Title and priority adjusted accordingly.

Work is in progress for creating packaging of nextcloud-client officially for Debian, which when/if succeeding (and not having Freedom issues) will get included in PureOS as well. Progress of that can be followed at https://bugs.debian.org/847613

Impossible to continue with owncloud-client.
I have a message : incompatibility wtih Nextcloud server 14.0.4

NB: at least as of 29 Jan 2019 Nextcloud-desktop is in Buster Testing.

Thanks for noticing, @Amgine.

@florent: Does that solve the issue you had?

Hello everybody,

It's a great news ! my own server on pureos !!!

Have a nice day