No static routes

Hey there, I just created my very first, own react app:

There is a sub-site:

The function works fine but I have an issue with the routing.
If you go first to the main page and then navigate to the sub-site per header link, everything works fine. But it isn’t working just by inserting the sub-site url directly to the browser. Someone told me it might be a server problem but not react.
I purchased my domain and the server on namecheap and publish my react-app by inserting the latest build in the ‘public_html’ folder.

Can anyone help?

Best regards from Germany