If you see a "Something isn't working. It looks like there was a problem on our end. Click 'Save' again" message, don't worry. This can happen if you're adding a QuickBooks Self-Employed client to QuickBooks Online Accountant.
When you sign them up, you start their subscription for them using a specific email. This message means their email address was already used for an existing QuickBooks Self-Employed subscription.
Reach out to your client to see if they already signed up. If they did, ask them to invite you to be their accountant. This automatically connects their account so you can review their books.
If they don't have a subscription, sign them up using a different email address.