When your assistant attempts to log into your Google account, Google may flag it as a suspicious login and attempt to verify that the account owner is the one logging in.
To authorize access to your Google account, log into it after the assistant has tried to. When you're logged in, you should see the following message at the top of your screen:
Warning: Google prevented a suspicious attempt to sign into your account. Was it you?If you tell Google that it was you, it should allow your assistant to access it. If your assistant is still unable to access it, please schedule a call with them so that they can go through the account verification process with you.