Plans
Mergify can be installed on stores and the app's settings can be modified and saved. But in order to have merging functionality, you have to activate a plan for the store. There are different plans available but the first one (Starter) is free to activate and 10 free mergings are provided to try the app.
When you activate a plan, its button will be disabled, and if you need to change the plan in the future you still can choose other plans.
note
The billing is calculated and managed by Shopify when changing a plan,