Hi there.

I replicated this on my end and was able to add the Check/Cheque number when creating a check for a supplier in QuickBooks Online. 

This type of behavior could be caused by a cache buildup on your computer. We can use troubleshooting steps to check and resolve any possible browser cache-related issues.

Use the keyboard shortcuts listed below depending on your browser:

• Google Chrome and Microsoft Edge - press Ctrl + Shift + N
• Mozilla Firefox - Ctrl + Shift + P
• Safari - Control + Option + P

If you're able to add cheque numbers, you can return to your regular browsing window and clear its cache. This will remove all outdated and corrupted internet files that are causing browser issues similar to the one you're experiencing.

Alternatively, you can look for and use other supported browsers that work perfectly with QuickBooks Online.

Let me know how this goes or if you still need my assistance with other QuickBooks-related tasks. I'll be happy to help. Keep safe!