Hi, Castle.
The error you encountered occurs when QuickBooks Online detects simultaneous edits on the same transaction or settings. This feature is designed to prevent changes from being overwritten when multiple users are working on the same data.
To resolve this issue and avoid it in the future, please make sure you have QuickBooks Online open in only one tab or browser window on your device. Additionally, close any sessions on other devices where you might be logged in.
If you have any further questions, feel free to leave a comment below.