Just felt I should make this post because I was nearly pulling my hair out trying to solve this issue.
I installed Google Tag Manager as specified in the documentation (Custom Code and on every page as a symbol) but I could not get the tag assistant to connect.
I had MalwareBytes installed as a Chrome plugin and removed it to try again.
It worked!
If you are having issues with Tag Assistant after installing your GTM container, I recommend checking your extensions to see if any might be interfering.