How can I connect an existing payroll core account to my new QuickBooks Online account?

Thanks for your reply.  I'm now running into this problem when I try to add a new employee:  "Cannot find employee for ID 0". I've entered all the payroll numbers and when I hit continue, I get this notification and it doesn't give me any way to remedy it.  I've had to enter this several times and each time I have to lose my data.

Let's make sure to fix the error that you are getting when running payroll, Hedgehog2.

In general, you'll only get the "Cannot find employee for ID 0" message when the number of active employees reaches beyond the limit. The hard cutoff should only be 150 employees. However, if you have lesser and yet you're still experiencing the issue, you can refresh QuickBooks by clicking the F5 button. This refreshes the currently active window and lets you work on a clean slate. Making it free from errors. 

Once done, ensure to review and enter all your employees' information. Sometimes leaving information fields blank lets you encounter errors and issues along the way when working with your payroll. You may want to make some changes to your employee profile by editing them. I've added this article for your guide: Edit or Change Employee Info in Payroll. 

In addition, QuickBooks provides several payroll reports that you can use to view useful information for your business and employees. Go through each list for more information: Run Payroll Reports.

This should let you put back on the right track. Keep me posted if you have any other concerns with payroll. Remember, I'll always be right here to make sure everything is taken care of. Stay safe always!