Hi, I am also facing the same issue - You go to Opportunity > Quotes (tab) > Click on "+ Add New Quote"

It will open up a the Quote form and all the fields will be read only and on the top the message appears like how mentioned above

And when you come back to Opportunity page, the Quote record is created but you can't do much about it (yes, you can select "Activate Quote" to mark it Active - but still it is not editable)

Note: This is a trial Org (setup on 1st April 2019) and I am logged as a System Admin while performing the above actions.