Hi there, AmyMac.

I have reviewed the steps provided by my colleague JenoP, it worked. I suggest contacting our Phone Customer Care to check your accounts and preferences or perform a screen sharing to verify if this need further investigation on why you're not able to create a sub-account in the Charts of Account.

Here's how to contact them:
 

1. Open QuickBooks.
2. Go to Help, then select QuickBooks Desktop Help.
3. Select Contact Us.
4. Give a brief description of your issue, then select Continue.
5. We’ll provide you a few support options. Select which one is best for you.

Also, please check our support hours and contact us.

Comment on this thread if you have other concerns. I'd be glad to answer them. Take care.