docs

1. The signup request failed because this organization has reached its active scratch org limit

Developer Edition org has limit for 2 active scratch orgs at the same time. You should delete one of them in order to create new one.
Using following command, you can check your active scratch orgs aliases:

sfdx force:org:list

Next command will delete scratch org, which you specify in argument:

sfdx force:org:delete -u <value>

2. Update (“refresh”) feature branch with changes from main

Sometimes when you develop in your feature branch, you may realize that main branch contains code or metadata, which you need for your feature branch. In this case, you may consider to refresh your feature branch with changes from main. Following command will merge main branch into your feature branch, but you should be aware of your merge conflicts, which may appear:

git merge origin/main -m "refresh" | git push

3. Fixing bugs

After merging your PR, you may realize that you forgot to add some changes or you have bugs in your code, which you need to fix. In this case you should create a new feature branch from main instead of using the same branch.

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4. Resolving a merge conflict on GitHub

Great article by Github how to resolve merge conflicts: here.