Microsoft project management software used to plan, manage, and communicate a project schedule and other information among workgroup members, project managers, and other stakeholders.
Hello Grant,
Thank you for posting to Microsoft Community.
According to description when you insert your work account to sign in for Project Plan 3 you receive an error message.
As Project Plan 3 requires a work account for activation and according to the screenshot you use a personal email.
When we talk about work accounts in Microsoft365, it might not be what you think at first. If your user name is ******@contoso.com it could be a personal or work account. You can’t always be sure just by looking at the email address. Even if you only use Microsoft credentials for work, your user name is most likely considered a personal account in this context.
This user ID becomes your initial Microsoft 365 email address, just to get you started quickly. This user ID is the email address that you use to sign in and if you don’t set your domain, it will end with “.onmicrosoft.com”. For example, if your business name is contoso, you might choose ******@contoso.onmicrosoft.com for your user ID.
For more information please check this article: What is my user ID and why do I need it for Office 365 for business? (microsoft.com)
If you are sure you use work account we may suggest you to contact the administrator in your tenant to confirm if he can sign up the Project license for you. For reference: How do I find my Microsoft 365 admin?
Feel free to share any updates with us so we can provide a further support.
Best Regards,
Igor