It's nice to see you in the Community. I'd be happy to help share the steps on how to add subtotals on the invoices.
At the top right, click the Plus icon.
Select the customer's name.
Enter the information.
Highlight the last row of the group, then click Add Subtotal.
After that, the subtotal will be inserted into the next row. Also, the following article contains additional information about this, as well as a video tutorial in creating an invoice: Create an invoice in QuickBooks Online.
For additional help, feel free to reach out to our Phone Support. The phone support agents will be able to assist you in getting this resolved.
That should get you on the right track for adding subtotals on the invoices. Keep in touch if you need more help with this. I'll be happy to help you out.