You may need to amend the sync settings for your Hike-Xero integration after setting it up, and here's how you can do it.

Please navigate to Hike Menu -> Integrated Apps, where you can see the MANAGE button in the row of your Xero integration.
Click on the MANAGE button and you will be able to view and edit your sync settings between Hike and Xero.

You may have the SAVE button greyed out, when your settings for the compulsory fields haven't been completed. It often happens when you activate a new feature or add a new payment type or tax after the integration is set up.

Please go through the sync settings page, and see whether you have complete the settings for all the fields having the asterisk. After completing the setup, you should be able to save the settings by clicking on the SAVE button.