Firewall from Cloudflare and native server Firewall

Hello folks

If someone can help me how to fix this .
I have very interesting and little bit strange thing about activating Firewall on my server.

At my server i have 2 sites with PRO plan from Cloudflare. So in that PRO plan those two websites are protected from DDOS and have Firewall.
Now,by default Firewall at my server is installed but not activated.

When i activate Firewall at my server with command
sudo systemctl enable firewalld
for rest of my sites , then i see message "host error" .
And when i disable firewall via Putty : sudo systemctl stop firewalld then everything back to normal.

Question:

Why when i activate firewall for my whole server via Putty, then my sites not work and i see message " host error"?

The point why i want to do this is:

I do not want to pay for rest of my sites on server Cloudflare PRO plan,and i want to protect other sites which are not on Cloudflare PRO plan,with native Firewall from server.

 

CloudFlare free are pretty enough as protector from DDoS. Activate one firewall only.
Firewall on server vs built-in CloudFlare Firewall will cause your host error.

 

So then i need to remove those two sites from Cloudflare,and again to activate server Firewall?

 

If your server support firewall and DDoS protection, there no need to use it anymore, BUT CloudFlare can help to save lot of bandwidth

 

Yes,that is clear. But when i enable firewall at my server then i got error: "Host error" and i can't see sites. When i stop firewall from SSH then website back online. Did this is a problem if my sites are pointed to Cloudflare and then firewall from my server can't work?

So i need to remove my two sites from Cloudflare who have Cloudflar's firewalls, and then i will not got "Host error" and firewall from my server will start to work right?

 

Is there anyone who can help me about my problem? Thanks