Lightning Experience and Salesforce1 do not support buttons and actions for Approvals
|Knowledge Article Number||000232861|
If you create an approval process on an object in Classic and then switch to Lightning Experience you will be able see the ‘Approval History’ related list however, you will not have the ability to use the "Unlock" button in Lightning Experience. Approvals are not fully supported in Lightning Experience or Salesforce1.
If you need to use the "Unlock" button please use Salesforce Classic. In the Classic user interface your users will be able to submit, recall, approve, or reject approval requests.