Hello @MichelleMM1! Thanks for coming back to the Community. I'm happy to help explain that setting. :)

The reason why you don't see that assign classes checkbox is because that feature is only available in QuickBooks Enterprise. There are a couple of workarounds that may work for what you're trying to accomplish. 

First, you can use the Class column instead and then select the right income or expense account when creating transactions like invoices, bills, etc. This way, you can link the correct class to its corresponding account properly. 

Second, you can add the class name on the account's Description field. You can also set a class if it's a sub-account.

Both of these options are explained in detail (with screenshots) in this related Community post: How to Assign Class to Ledger Account. Scroll down to the agent ReyJohn_D's comment where he outlines each option. 

Please feel free to reply to this post if you need anything. We're all eager to help you!