I can help you with resolving the error you're having when adding your invoice, @zamboknee
It's possible that there are some minor data issues with your company file that's why you're having errors with your invoices. Try running the Rebuild and Verify Utility tool to fix possible data damage on your company file.
Please follow the steps below:
Open your company file again.
Go to the File menu, then select Utilities and choose Rebuild Data.
Click OK in the QuickBooks Information window.
Wait until the repair is done. Then, click OK.
Then, go back to the File menu and select Utilities and choose Verify Data.
Click OK if QuickBooks doesn't find any problems. Or Rebuild Now if it finds issues with your company file.
If you still get the same issue, let's try running the QuickBooks File Doctor to fix common issues.
Open your QuickBooks Tool Hub.
Select Company File Issues.
Click Quick Fix my File.
Choose OK when it completes and launch your QuickBooks.