I just started using GitHub Copilot.
https://copilot.github.com/

Copilot shows suggestions when I write code, but the autocomplete functionality doesn't work with tab key.

I could find out the reason on GitHub Copilot discussion.
The reason was Emmet.
Recently I don't use Emmet heavily so the solution was very simple.

Solution

Open settings on VSCode and turn off Emmet:Trigger Expansion On Tab

In terms of GitHub Copilot, so far so good!