We can perform some troubleshooting steps to add a company to your QuickBooks Online (QBO) subscription, Kaium.

Have you encountered an error message when adding a company file to your QBO subscription? To successfully add a new one, sign up for a new QBO account. One company file is equivalent to one subscription. Here's how: 

1. Go to https://quickbooks.intuit.com/in/pricing/.
2. Select Try 30days Free Trial.
3. Below "Already have an account?", select Sign in.
4. Sign in with your previous or current user ID and password for other company subscriptions. Follow the onscreen prompts to set up a new subscription using your existing login.

To put this company under your existing Intuit ID, log into your current company when creating a new account. Then, click the Add another company tab when creating another account. From there, you'll only be asked for an Email address and Password ( use the same email address). You can check out this article to help manage multiple company files: Create or add another company file to QuickBooks Online. 

If you are using  QBO Accountant and you want to add the client's company to your firm, refer to this article for the complete details: Add clients to QuickBooks Online Accountant.

Moreover, you can check out this article to easily switch between QBO accounts: How Do I Switch Companies?.

If you have further questions about managing your QBO company files or QBO subscriptions, add them to the thread. We'll be glad to provide additional assistance, Kaium. You have a good one.