Hi KrisMc,

Thanks for posting in the Community!

There's no reported incident with the error you're getting when trying to click the Customise look and feel under Company settings. I replicated as well in my end and is working fine.

Let's perform some basic troubleshooting steps to isolate the behavior encountered in your QuickBooks Online account. To start with, use a new incognito window or a different supported browser. Using an incognito window does not store any data file.

Here's how:

• Google Chrome: Ctrl + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Safari: Command + Shift + N

Then, log in to QBO and access the Company settings once to customise your sales forms.

If it works fine when accessing the new incognito window,, I would recommend going back to your default browser and clear your browsers cache.

If the issue persists, kindly use a different supported browser or a different device to isolate the behavior encountered.

Post again in the Community if you have further concerns. I'll be around to help you.