Hi there, @Dorian11.
The transaction date prompt messages stopping you from creating new entries is related to the closing date of your accounting books. Let's go over and discuss this further.
In QuickBooks Online, one must select a closing date before the fiscal year ends, preventing any changes to your financial data and keeping the accuracy of your accounting books. Mainly the reason why it's not possible to enter new purchase orders with previous year dates or older than the current fiscal year.
Kindly read and use this article to learn more about the closing date in QuickBooks: Close your Accounting Books in QuickBooks Online.
Saying you're about to enter these purchase orders for practice tasks related to a course assignment, we can use the QuickBooks Online Test Drive link instead. This test drive ensures you got what it takes to use the features in QuickBooks, allowing you to modify entries on any dates you wish to enter them.
Therefore, here's the article you can use the learn more about the test drive: Test Drive Links in QuickBooks Online.
Now on a fixed date set after creating your purchase orders which are non-posting entries, you'll be asked to convert these entries linking them to your vendor's account. I have here the reference with the steps you just needed: Add Purchase Orders to Expenses, Bills, or Checks in QuickBooks Online.
I've got you covered if you have questions other than entering purchase orders. Don't forget to include my name, @JonpriL, in your reply in the comments below so that I can get back here and answer all your QuickBooks concerns.
