Installation child website is not going well

I have made a special wordpress installation for mainwp. I have already been able to link a number of websites so now I try to link vacuload.com but I get an error message.

this is the message:
MainWP Child Plugin not detected, first install and activate the MainWP Child plugin and add your site to MainWP Dashboard afterwards. Click here to install MainWP plugin (do not forget to activate it after installation).
Hint: On your child’s site, go to the Server Information page and verify that the SSL extension is enabled.

I just installed the mainwp child plugin, I also have a ssl certificate and the simple ssl plugin is on.

I’ve already tried everything and don’t do anything else.

How can I solve this?

The page I need help with: [log in to see the link]