Hello there, ssg7741.

Welcome to the Community. I'm here to lend a hand with your concern about signing into your Intuit account through QuickBooks Desktop.

Before we start, I want to make sure you'll be able to start a new discussion here in the Community. You can try following these steps:

1. Go to: https://quickbooks.intuit.com/community/US-QuickBooks-Community/ct-p/community-us.
2. Click QuickBooks Q&A.
3. Next to QuickBooks Q & A for, choose QuickBooks Online.
4. Select a topic.
5. Click Start a discussion.

If you're still unable to post a discussion, you can try logging into a different supported browser to determine if the problem is website related or not. You can check the screenshot I'm adding down below for your visual reference:

Now let's start with your concern about the existing Intuit account. Since you're unable to change the previous email address attached, we can try updating the information through the My Company section.

Here's how:

1. Click the Company tab at the top menu bar.
2. Select My Company.
3. Click Manage Your Account (or the existing email address). 

For additional reference, I'm also adding the article I recommend about updating email address:

Update email address.

Please know you can always come to the Community for any assistance you need with connecting your account. You can reach out to me by posting your response below.