If you cannot enter in your SSN/EIN on your W9 popup, you will first need to update the information listed on your Profile page.
Within your back office, click on your name in the top left corner. A menu will appear, click on Profile.
You will see the SSN/EIN field listed on this page. Click the Blue pen/paper Edit button and re-enter your ENTIRE SSN or EIN (depending on whether you are set up as personal or business).
Once you have re-entered your entire SSN or EIN, click the GREEN Save Changes button in the top right corner.
The W9 will then popup right away and you can now enter in your information then sign it to submit to Accounting.