Hello, bgaudiovisual. This occurs when you select Out of Scope under the Sales Tax settings on the invoice page. Please see the screenshot below:

To ensure that GST is applied correctly, select either 'Exclusive Tax' or 'Inclusive Tax'.
If you have already selected the appropriate tax settings and GST is still not calculating, a quick data refresh can help get everything back in sync. You can follow the steps below, depending on your device.
On iOS:
- Go to Menu ☰.
- Press Settings, then Refresh Data
- Tap Refresh Now.
On Android:
- Go to Menu ☰, then tap More Options ⋮.
- Tap Settings, then Refresh Data.
- Press YES to confirm.
Alternatively, uninstall and reinstall the app to ensure you're using the latest version.
If you need further assistance, please leave a comment below.