Unfortunately, the new REACH NOW app does not allow you to book public transport tickets directly for now.
In Stuttgart, however, you can book tickets with the SSB BestPreis app. If you search for a connection in the REACH NOW app, you can book it via a link to the SSB BestPreis app. An additional user account is not necessary; your REACH NOW account and payment method are automatically stored in the SSB BestPreis app.
Please note that SSB tickets are purchased through the SSB BestPreis app and therefore can only be found in the SSB BestPreis app after purchase.