I'm here to ensure you can add your client to the program successfully, @nacrisgll.
The Add client tab is only available for QuickBooks Online Accountant subscriptions. If you have multiple companies, make sure you accessed the correct one. Please know your client can also send an invitation link to the email address you use for your QBOA firm.
Moreover, we can perform browser troubleshooting steps if you're using the Accountant version. Doing so will help us verify if this is a webpage-related issue. To open an incognito or a private window, refer to these keyboard shortcuts:
Ctrl + Shift + N for Google Chrome browser
Ctrl + Shift + P for Mozilla Firefox & Microsoft Edge browsers