When you select page the URL Parameters for Site Pages, in that page you write REST API jquery and after adding in script editor if you save page API does not work after saving because of Minimal Download Strategy Site Features Active.
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After adding in script editor Before saving Page:

Read some SharePoint 2013 tutorials below:
- Bind SharePoint list items to dropdownlist using Rest api and jQuery in SharePoint 2013
- SharePoint 2013 Get current login User Profile Properties through REST API
- Upload large files to SharePoint online document library using PowerShell csom
Proposed Solution :

To validate if everything was working correctly, deactivate the MDS Feature and access to a SharePoint Page.
Feature go to site Settings -> Manage Site Features – > Find Minimal Download Strategy Feature – > Click on Deactivate.

After access the SharePoint page you will be able to see the URL change, it’s not calling “_layouts/15/start.aspx#/” but the correct Page “/sitesPages//POCForDropDown.aspx” and the Ribbon Load correctly.

Hope this article will be helpful.