Hi there kc918,
It's important that all your transactions are correctly entered so that your books can be well balanced. QuickBooks Online is a great tool to help you keep track of sales and expenses so you can see the profit of your business. I can point you in the right direction for assistance with custom fields.
If one of your companies doesn't have the ability to add custom fields to your suppliers and customers, I recommend reaching out to the QuickBooks Online support team outside of the Community. They'll be able to view both accounts with you in a secure setting to see the difference in settings to see why one company can add custom fields to a supplier/customer, and the other can't. You can reach them using these steps:
Schedule a Callback or start a Chat: click (?)Help in the upper right > type and enter "Contact support" into the QB Assistant > click Contact Us > explain your situation > click Let's Talk > choose Get a callback or Start a Chat.
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If you have any other questions, feel free to reach out here.