I know the possible reason why there's no longer an option to process prior period payrolls on your specific employee, flemingbd35. Let me elaborate on it in detail.
Keep in mind that you can modify your pay history if you haven't generated any paycheques. In your situation, it's likely that you've already created one, which is why you're unable to make changes.
In the meantime, you can reach out to our QuickBooks Live Support team for assistance with making adjustments to your payroll history. They will be able to guide you through the process and ensure that any necessary changes are accurately reflected.
Here's how:
- Sign in to your QuickBooks Online company.
- Click on the (?) Help icon.
- From there choose the Assistant tab to get help from the QB assistant, or select the Search tab to enter your question and find Help articles.

For more detailed information on how to contact our live support, you can check this article: Get help with QuickBooks products and services.
Additionally, for further assistance, you can refer to this article to discover how to set up extra pay items in QuickBooks Online Payroll and manage pay types that you no longer use: Add or change pay types.
I'm still ready to back you up if you need more help with payroll. I'll keep the thread open so you can comment back.