Your Employer Account Number can be obtained directly from your state tax agency. I'm here to share a few insights and help you from there.
Firstly, your Employer Account Number has a unique format (999-9999-9 or 999 9999 9) which will help you determine if it's one or not. Also, your state tax agency only knows and will give you the exact digits of your Employer Account Number.
Thus said, you can visit our California Payroll Tax Compliance page to learn more about your Employer Account Number. In addition, the ways of contacting your state tax agency are outlined in the said article.
Lastly, you can always visit our Help Articles page for QuickBooks Online Payroll in case you need some easy steps for your future payroll tasks.
If there's anything else that I can help you with, please let me know in the comment section down below. I'll be always around ready to help.