The regular 19.1 lacks the code to update from 1.6, that code is in the old phpgwapi repo, or egroupware-epl-compat package.
You still managed to not give an answer
I can only assume you used the archive from our GitHub release page, to get the sources.
If that is true, you have to git clone the phpgwapi repo, into this sources:
git clone https://github.com/EGroupware/phpgwapi.git
BEFORE you run the update!
So you have to restore the 1.6 again, after the above git clone, and then run the update again.
Probably needless to say: archives are not a recommended way of installing EGroupware!