Thanks for reaching out to us, Charlie.

You can use the class feature in QuickBooks Online to get deeper insights into your sales, expenses, or profitability for each part of your business. Once it's turned on your can easily assign classes to each item or transaction.

If you want to create or update an existing item and assign a class to it in QuickBooks Online, make sure the class feature is enabled. I'll show you how.

1. Go to Settings ⚙ and select Account and Settings.
2. Click Advanced, then select the Categories section to edit.
3. Select the Track classes radio button.
4. Checkmark the Warn me when a transaction isn't assigned a class. This is optional.
5. Under Assign classes, choose either One to entire transaction or One to each row in transaction.
6. Hit Save, then Done.

After that, you can create and set up a class list to assign classes to your items or transactions.

If you wanted to create or update an item and assign classes to it through Integromat, I suggest reaching out to their support team. They're familiar with the software and they'll walk you through the steps.

On the other hand, I encourage you to visit our Intuit Developer's website for additional help with the error you receive when attempting to use the Make an API call option when assigning classes to your item. You can submit your questions by logging in or creating a new account from there. After that, someone from the developer's team will respond to any questions or concerns you may have.

I'm also including this helpful article for setting up and tracking the item in QuickBooks Online for future reference: Add product and service items to QuickBooks Online.

Please let me know by adding any details below if you need further assistance in QuickBooks. I'll be around to help you.