Getting Error while adding an app to the Classic sites of SharePoint online

Hi,

I am getting an error (Sorry, something went wrong) while adding an app to the classic SharePoint site.

I have tried to add multiple apps, all apps are giving me error.

These apps were working fine few days ago, we didn't make any changes in our tenant.

If I try to add same app to the Modern sites, it's adding perfectly.

I am not sure, how to resolve this issue. Any help is much appreciated.

Thanks, Pooja