I guess you're attempting to play it through single player, as you can't really play with the current present day Quake TF community as they use special Quakeworld servers that require better, bugfixed Quakeworld clients not available for DOS (no, that wattcp stack port probably wouldn't work)
Extract tf28.zip manually to a folder called fortress/ in your Quake folder
Run DOSBox and go to your quake folder in there
quake -game fortress
you should see the intro sequence.
If you SERIOUSLY want to play it online, just grab the latest Darkplaces, extract that into your Quake folder, and find a quakeworld "fortress" server. It'll autodownload the rest 😁

long live PCem
FUCK "AI". It is a tool of fascism. We do not need it. We do not use it.
Debian's fucked