Hi MMZS,
You might have to check your QuickBooks Online subscription version. The Custom field option is currently available in the PLUS and Essentials versions of QuickBooks Online. If you wanted to use the Custom field option you might want to upgrade your subscription to the Essentials or PLUS version.
You can check this link on how to Upgrade or downgrade your QuickBooks Online subscription and how to Add custom fields to sales forms and purchase orders in QuickBooks Online.
Post again in the Community if you have more questions.
