Hello Samantha,
Welcome to the Microsoft community.
According to this blog post, I noticed that Microsoft Designer is mainly available for Microsoft 365 Personal and Family subscribers.
New tools from Microsoft 365, Edge, and Bing | Microsoft 365 Blog

Since the section where you posted the thread has been moved, I'm wondering if you and your colleagues are logged in using personal Microsoft accounts?
I also tried to join the Microsoft Designer waitlist with a personal account and a business account, and I was able to join the waitlist with both accounts to gain access. However, when I logged in with the business account, I got the same error message as you did. As you can see, Microsoft Designer is not yet open for enterprise users, so although you have access, you will still get an error when logging in with a business account. At present, Microsoft Designer is still in the stage of exploration and development, and it will be gradually opened to all users.
Currently, if you want to use Microsoft Designer, you may need to use your personal Microsoft account to join the waitlist and access it.
Feel free to post back if you need further assistance.
Sincerely
Mavis - MSFT | Microsoft Community Support Specialist