Please enter a commit message to explain why this merge is necessary, especially if it merges an updated upstream into a topic branch

It’s not a Git error message, it’s the editor as git uses your default editor.

To solve this:

1. press “i” (i for insert)
2. write your merge message
3. press “esc” (escape)
4. write “:wq” (write & quit)
5. then press enter