After Git10 update fork Code

First you have to ascertain whether the establishment of a remote source repository:

git remote -v

If you can only see your own inside two sources ( fetchand push), you would need to add the source of the main repository:

git remBASHote add upstream URL
git remote -v

Then you can see the upstream.

If you want to merge with the main repository:

git fetch upstream
git merge upstream/master