I'd like to help you get this sorted out. Are you trying to send an invoice and then received the error message? If so, let's perform some troubleshooting steps to get you back in working order.
I suggest using a private or incognito window when sending an invoice from your QuickBooks Self-Employed (QBSE) account. This way, we can check if this is a data-related issue. Over time, data stored from the browser causes unusual behavior. To open a private or incognito window, you can follow these keyboard shortcuts:
Google Chrome: Ctrl + Shift + N
Internet Explorer & Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Option + P
Once in a private window, try to send your invoice. If it works, then we'll need to clear your regular browser cache. This will give your browser a fresh start.
If the issue persists, try using another browser. This way, we can see and check if this is a browser-related issue. Additionally, if you're using QBSE mobile app, I recommend reinstalling and installing the app.
However, if you're referring to something else, you can always get back to us anytime so we can address it right away. Have a great day and stay safe.