Theme Child

Hi there,
i am working with a theme child on a wordpress multisite. An then today i notice a error that i dont understand because i always worked on child theme and the site is on .In FTP everything is ok and the folder child theme ok but in the wordpress shws an error . I wanted to use theme child in another site on multisite wordpress and soent show.

I dont want to lose nothing on the first site because is on and it is from a political party with a lot information and a lot of work that i made in theme child.

Can you help? i dont want to lose anything all the work is online and it cant be lose now.