Welcome to the Community, @the-rahhh, and I'll help you fix this right away.
The ability to add a discount can normally be accepted when saving a bill using a mobile app. Also, it will calculate the amounts in CGST and SGST columns accurately. In this case, let's make sure that your mobile app is in the latest release. This way, we can avoid any issues with the invoice.
Once confirmed and the issue keeps going, let's get your mobile app up and running by resetting the QuickBooks Application. Having a lot of cache in your mobile phone can cause the item to misfunction in your bill.
Here's how for Android:
After that, I'd recommend adding the discount on the bill again to verify if it's saving the transaction and calculates sales taxes correctly. If the issue continues, let's uninstall and re-install the app. Please touch and hold the app, then click the Delete app or Uninstall button. Uninstalling won't remove data since it's secured and stored in an online server.
Once done, you can now reinstall the app. For the detailed steps, please browse through this article: How to download the QuickBooks Online mobile app.
I'd also encourage you to check out these articles about mobile app and device access.
You're always free to post a comment below if you have additional questions about creating sales and expense transactions. I'll be more than happy to assist you. Stay safe!