This might be a temporary issue with the authentication on Google, but in any case, please do try the suggested workaround below:
Open Insync, click the (+) sign on the upper panel to add your account. Or if you’re adding the first account, select “Add Google Drive”
A browser will launch. Copy the full URL and then open an Incognito Browser (Ctrl + Alt + N).
Paste the URL into the Incognito browser. It’ll ask you to enter your email and password. Proceed with the rest of the steps to complete adding your account on Insync.