Yes. You can always choose to reactivate the Plugin. However, there are 2 possible scenarios here.
Scenario 1: You have deactivated the Plugin and want to reactivate it BEFORE the end of your current billing cycle. In this case, you just need to put a tick on the checkbox on the “Tagrain Xero Integration” Plugin Tile and the Plugin will get activated.
Scenario 2: You have deactivated the Plugin and want to reactivate it AFTER the end of your current billing cycle. In this case you will have to again subscribe to the Plugin through the normal subscription process.