Cannot configure the Content data source, received warning "Please configure the data sources used."

I try to configure the Content Data source under editing the "Search index Default table of contents". I received a warning saying: "Please configure the data sourced used" and my settings are not saved. I am using D10.1.4 with Search Api 1.30.

What should be the exact steps in order to achieve this?
I want to have the option "DO NOT include this content in the search index." for the new content data.

Thank you!