Perhaps this helps >>> sharepoint enterprise - Break role inheritance and assign permissions to document with REST api? - SharePoint Stack Exch… 

Hi Fernando,

Yes, I tried this one and getbytitle also. In both the cases I get Forbidden error message. I also tried to use the action 'Update Item Permissions' and that too doesn't seems to be working.

Hi Taurundeep,

Who is the initiator of the workflow, generally the API request made from the workflow will be executed as the initiator of the workflow. Do they have access to perform permission modifications on this item? Alternatively using the request inside of an Action Set with elevated permissions enabled may help?

Thanks,

Callum

Hi Tarundeep Singh, you need to elevate the permissions using App step. 

1. IN Manage Site Feature, activate feature "workflow can use app step".
2. Now go to Site settings -> Site App Permissions , there you will find an entry for Workflow as i:oi.t|clientID@tenantID.
3. Copy the Client ID and give it full control over site. To know how to give workflow full access please follow this blog. SharePoint Blog: Item Level Permission - SPD Workflow 2013 - Part 2 . follow same steps as mentioned for SPD workflow.
4. Now In Nintex workflow, wrap your http action in Action Container and allow it to run as an app step.
5. This way you will be able to do your task. 

Hi Tarun,

Hope it will help you. 

1) Nintex App Permission

Create a workflow with elevated permissions by using the SharePoint Workflow platform | Microsoft Docs 

2) Add REST in Web Service Like below

SitrURL +      "_api/web/lists/getByTitle('Documents')/breakroleinheritance(copyRoleAssignments=true, clearSubscopes=true)";