The inventory start date is the 20th, and the product was invoiced once on the 21st, today it’s giving me an error asking to choose a different transaction date.

Let's get rid of the errors you can create an invoice, bamfoods1-gmail.

There's a possibility that the inventory was adjusted and the Starting value was changed. You can verify this by opening the item and check the date. 

Let me show you how:

1. Go to Sales, then Products and services.
2. Find the item, then click Edit under Action.
3. Select Starting value, then Got it!
   
4. This will open the Inventory Starting Value page.
5. Check the date in As of date field so you'll know when to enter the invoice.
   

Once verified, you can make changes to the start date when needed or adjust the date when you create the invoice.

I'm adding this link to help manage your products and services: 

• Set up and track your inventory in QuickBooks Online
• Adjust inventory quantity on hand in QuickBooks Online.

Keep me posted if there's anything else you need about your items. I'm always around to help manage your QuickBooks.