tlmgr is the TeX Live Manager and responsible for the TeX installation on your (Linux) machine.

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If you're getting the message:

(running on Debian, switching to user mode!)
cannot setup TLPDB in /home/dominik/texmf at /usr/bin/tlmgr line 5336.

... tlmgr has not been initialized. Run this to initialize it:

tlmgr init-usertree

Then, you may set e.g. a global paper size standard: tlmgr paper a4.