Hi, dominic14. Accumulated cache or disruptions in online components commonly cause errors in QuickBooks Online. To better address your concern, could you clarify the following:
What transaction are you trying to record in QuickBooks Online?
What exact error message are you receiving?
What troubleshooting steps have you already tried?
In the meantime, here's what you can do to start troubleshooting:
Log out of QuickBooks Online, then log back in to refresh your session.
Refresh the page by pressing F5 on your keyboard or clicking your browser's Refresh button (⟳).
Open QuickBooks Online in a private or Incognito browser window to bypass cached data.