Good morning, @WanderJax.
I appreciate you stopping by the Community!
I can see how beneficial this could be for you. However, there isn't a way to make the Class field a required step to save or move forward on invoices or sales orders.
I'll send your feedback over to our Product Developers so they can consider this in the near future. As a future reference, check out this guide below:
Set up and use class tracking in QuickBooks Desktop
Feel free to reach back out if you have any other questions. Take care!