This may be somehow related to either your browser and its settings/extensions or perhaps your MainWP installation and its plugin.
Because, even when the information entered in step 7 is completely random, and you click the Connect Account button, you should be redirected to Google sign-in page and get an Authorization error.
Can you try doing the same process again from start, but do so in an Incognito/Private browser window without any browser extensions, and try disabling all other plugins on the Dashboard except for the MainWP Dashboard and Google Analytics extension?