SharePoint access reqeuests - unable to pick groups

Hello,

I noticed that for a site collection I'm unable to pick the group I want the requesting user grant access to. Usually, it looks like this:

However, this "Permission" drop-down menu does not appear for the site collection in question.

Any idea why that is? First I thought that a default group was not defined but even after doing so this drop-down doesn't appear.

Thanks for your help.