How can I use Tab-Completion in PuTTY connecting to a Ubuntu Server?

Tab completion should work in PuTTY. Tab completion is provided by the shell you're running on the server, not by PuTTY itself. Please edit your question and provide more details including what shell you're using and what specifically happens when you try to use tab completion.

I had an interesting variation of this, where it worked for root, but not for my personal user. It's a local VM, as krowe suggested, on a VirtualBox running Debian. I used the same connection, so the issue had to be on the server side.

As it turned out, the difference was that root had /bin/bash in /etc/passwd, while the personal user had /bin/sh.

Changing the default shell to /bin/bash solved the problem for me.

I have the same problem running Putty on Windows and sometimes, hitting Tab just takes the focus away from the Putty window and does not do the autofill that it usually does under /bin/bash. I have tried all of the solutions mentioned above and none of them seem to fix this problem for me.

This is how I fixed mine. Go to the Windows Desktop screen and press the Alt-Tab and Ctrl-Tab combination keys a few times and that seems to fix it for me. Now go back to the Putty screen and try Tab again.