No, it's not a syntax error; it's just a letter which is appended after the expansion of $PATH, because the shell removes quotes...

$ PATH="$HOME/bin:$HOME/.local/bin:$PATH"i

$ echo $PATH

/home/zanna/bin:/home/zanna/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bini

So, as well as prepending local directories, it has effectively removed the existing /snap/bin from my PATH, and added the non-existent /snap/bini.

You can remove the i to repair your PATH.

To see the change, you will need to log out and back in or run source ~/.profile in any shell you are using (or launch the shell with bash -l), because .profile is read by login shells only.

If you did not make this change to your .profile yourself, you may want to restore the default file by running

mv ~/.profile{,.old}

cp /etc/skel/.profile ~/.profile

This renames the old .profile .profile.old (you could also delete the file if you wanted to) and replaces it with the default version for your system from /etc/skel.