Hello there, Karl. I'd be glad to share with you the steps to help resolve the error you're getting when creating a journal in QuickBooks Online (QBO).

Since you're able to ensure that the name you've added in the Name field in line with Trade and other payables accounts is a supplier, I recommend performing some troubleshooting steps as it could be related to your web browser. 

To start you can open your QBO account via a private browser to have a clean slate. This browser mode does not store cache that might be corrupted and results in experiencing odd behavior when running web-based programs.

When ready, refer to the shortcut keys below depending on your browser type: 

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

Once signed in, you can now create a journal again and ensure to assign the correct supplier in line with the Trade and other payables. If this works, I recommend clearing the cache of your regular browser to eliminate the piled-up data that may cause issues. If not, you can use another supported browser or device. It might be that the current one you're using has a temporary issue with the program. 

For reference, check out this article: Create journal entries in QuickBooks Online. 

Should you need additional assistance or anything you'd like to ask about managing your journal transactions in QuickBooks, please don't hesitate to leave a reply below. The Community is always open to back you up in resolving your concerns.