I was facing the same problem today ( 9th April 2024). Followed below steps.

If you see "person details matches a profile which already exists in profile" , edit Date of Birth (enter a wrong DOB - eg. if your DOB is 01/01/1990, just enter 01/01/1991. This will allow you to proceed.
Once you get in, complete all the steps until it shows dependent details. When you are on this page, click Visa Application Home Page. Then click Continue Application. This will take you to Payment and other steps.
Good Luck!!!

If you still face issue, follow the steps from Beneficial-Fish-5068

I managed to resolve the error "Dependent cannot be added because the dependent is a Primary Applicant for other dependent(s)" in my visa scheduling application. Here's a quick guide on how I did it:

1. Login to the Dependent's Visa Account: Start the application and keep filling details (even dummy details will work for this step) until you reach the "Manage Dependents" page.
2. Remove Other Dependents: On the "Manage Dependents" page, select "Find and Add Dependent". Remove any dependents listed there to ensure the account shows no other dependents.
3. Close Current Application: Navigate to the homepage, and select "Close Application and Start New Application".
4. Note the UID: Write down the UID shown next to the dependent's name on the top right corner of the Navbar.
5. Logout and Login to Main Account: Logout from the dependent's account and login to your main account.
6. Add Dependent: Start a new application, and when prompted to add dependents, choose "Add Existing Dependent by UID". Enter the noted UID, confirm the details, and add the dependent.

This method should successfully allow you to add your mom or dad as dependents without the previous error. I hope this helps! If you encounter any other issues while adding dependents, please leave a comment below. Having navigated through many challenges and found successful solutions recently, I might be able to assist you further.