The Credit Limit field is only available in QuickBooks Desktop. As a workaround, you can enter it in the Notes field in QuickBooks Online.
I'll go ahead and make sure to send this topic as a product suggestion to our Product Developers.
Tell us more about your experiences in QuickBooks Online. You can post them here.
I can see how important having a customer limit is for your setup with QuickBooks Online. Our goal is to provide QuickBooks users with the features that can help them best manage their books. At this time, however, the customer credit limit hasn't been added to the program.
I recommend considering the workarounds suggested in this thread, such as using the Notes field of the customer profile to document this information or a third-party app. You can review and research the available apps through the Apps tab right in your books and easily connect through there if you find one you like for managing your customers.
If you haven't already, I also suggest submitting your feedback about this feature. Although others have as well, it doesn't hurt to have more requests and it helps your voice be heard. Here are the steps to take to leave feedback: How do I submit feedback?
Feel free to ask more questions if you have them. Enjoy your day.