Why can't I find any manpages?

I guess something is wrong with your manual pages caches, run:

sudo mandb

to update it, if it does not exist on your system it's going to be created.

You can also use sudo mandb -c to fore the removal of old cache and creating a new one.

The other option which may be useful to you is -t, it perform correctness checks on manual pages.

If you have some packages installed locally, e.g using pip or other package managers in ~/.local/bin then to add their manual pages once again run it without sudo:

mandb

---

Extra steps

If the above instruction did not worked for you use manpath to find out about manual search path, make sure it's not empty.

Then check to see if any manual has been installed at all:

$ man -w man
/usr/share/man/man1/man.1.gz

If the command does not have any result use:

$ file /usr/share/man/man1/man.1.gz
/usr/share/man/man1/man.1.gz: gzip compressed data, max compression, from Unix

Then try man to open the manual, see if it works at all:

man /usr/share/man/man1/man.1.gz

If you didn't find out any man page try reinstalling that package.