I rechecked on the server. There is no folder named api 
I used the “manual way of installation” via Github as described here: https://wiki.dreamfactory.com/DreamFactory/APT/Ubuntu_16.04/Installation, although my server runs debian 9 instead of ubuntu. I also deviated from the description by using php7.2 instead of 7.1
All other steps did not show an error. The database was correctly initialized and I do have a user in it.
So which step of installation do I have to repeat, to create an folder /api with all the necessary content?