Why getting this error tlmgr Unknown directive?

If you want to continue to use 2015 version you should set the repository to the archived version with:

tlmgr option repository ftp://tug.org/historic/systems/texlive/2015/tlnet-final 

as is explained here How to install a package from an older version of TeXLive?.

You are using tlmgr from TeX Live/Debian, that is the packaged version. This version does not work with current (2016) release, and there is no way around that.

You have two options:

• install upstream TeX Live
• stop using Debian's tlmgr to install packages unless you find an archive of TL 2015 version.

This should happen only rarely when we (TeX Live Team) need to change the database format (it didn't happen in the last 5 years or so, and hopefully it will not happen for more than 5 years).

You can not upgrade TeXLive 2015 to TeXLive 2016. You should do a clean install.

From here:

Upgrade from TeX Live 2015 to 2016

Due to changes in the infrastructure, it is not feasible to upgrade from the TL'15 release to TL'16. (Ditto MacTeX.) Please just do a new installation.