Update Problem

Hi
i have just followed the pinned post :
1. change active theme to other one
2. delete old parent theme
3. upload new parent and when i try to activate it i get this error and whole web crash.
https://gyazo.com/bfc3a2c393c37129ab7bba8b65c51585
i have back up of the website
but what can cause it to happen? im using child theme but also tried to activate the child instead the parent and the same error appeared

Thanks