Help, tpm not working!

Here's the list of issues users had with tpm:

Nothing works. tpm key bindings prefix + I, prefix + U not even defined.

Do you have required tmux version to run tpm?
Check tmux version with $ tmux -V command and make sure it's higher or equal to the required version for tpm as stated in the readme.
ZSH tmux plugin might be causing issues.
If you have it installed, try disabling it and see if tpm works then.

Weird sequence of characters show up when installing or updating plugins

This could be caused by tmuxline.vim plugin. Uninstall it and see if things work.

"failed to connect to server" error when sourcing .tmux.conf