Hi alisonfromulverston, thank you for posting here in Apple Support Communities. We understand that you're no longer able to use a specific third-party on your Mac computer since updating the macOS, and we'll definitely do all that we can to help. To get us started, can you tell us if you've checked the Mac App Store to see if there are any software updates for that app? Use the App Store to update apps on Mac - Apple Support
If so, we'd then recommend that you check the app's developers website for additional information regarding its compatibility with the latest macOS software. Here's how: Free Download. Invoice Software for PC/Mac. Generate invoices, quotes and orders.
In addition to that, you can also check out the following alternative invoicing apps available for macOS:
GrandTotal - Invoices and Estimates on your Mac
Orbit: Time-based Invoicing on the App Store
We hope this helps.
Cheers!