Multisite Theme-Upload Failed

Hi There!

I got a problem with my WordPress Multisite. I am in the Network Admin Theme Section and Click on Add Theme and then I want to choose a Zip File to upload. After clicking the “Upload” button there’s a message: “Are you sure, you want to do this?” with a link back to the Theme Section. I already set up the multisite installation new, but it still doesn’t work. Already googled and searched the whole internet, but couldn’t find a possibility to fix that problem.

I want to install AVADA theme. But it can’t be that i can only upload themes by ftp. I want to use the WordPress functions.

Can anybody help?

Greets,
Domenic