I just went through this and the support rep at QBO could not figure it out, but my husband had an idea and it worked!
Here are the steps:
1. Click "gear" icon in upper right corner
2. Click "Account Settings" in upper left column
3. Click "Sales" in left column
4. Click into "Sales form content"
5. Turn off "Custom transaction numbers."
6. Click "Save"
7. Click "Done"
At that point I logged out of QBO and logged back in.
I then repeated steps 1 - 4 and in step 5 I turned "on" ""Custom transaction numbers." Then I hit save and done.
Then, I created a new invoice with my new desired invoice numbering system. I created 2021100, 2021 for the year and then 100 as a starting point. I have a "Test Client" set-up so that invoice was sent to my Test Client.
Then I opened a new invoice and sure enough the invoice number was 2021102, voila, success!
