Let's resolve the error you're having to make your customer inactive seamlessly, @LWPFL.

Since you're still having an error after trying those troubleshooting steps, I suggest clearing your browser's cache. 

It's possible that the piled-up cache in your regular browser caused this unusual behavior. 

Go to your account by using a private browser or an incognito window, then try to make this customer inactive. If it works fine, clear the cache in the one that you're currently using.

Here are the following keyboard shortcuts: 

• Safari: Command + Shift + N
• Mozilla Firefox: CTRL + Shift + P
• Google Chrome: CTRL + Shift + N 
• Microsoft Edge:  CTRL + Shift + P

If it works, here's how to clear cache and cookies to fix issues when using QuickBooks Online. Restart your browser right after. 

You can also proceed by using other browsers if the steps above don’t work.

Refer to this article for more information about removing a customer: Add and manage customers in QuickBooks Online. It will also guide you with adding, editing, deleting, or merging customers. 

Let me know if there's anything else that I can help with your customers. I'm always here to help. Have a wonderful day!