While creating a web application, by mistake selected “Windows” authentication, and felt we need “Forms” authentication (Claims Based Authentication) because of a business requirement.
Went to Central Administration >> Application Management >> Manage Web Applications >> Selected the desired web application >> Authentication Providers.
Oops, “Forms” option is disabled! Couldn’t find any way to enable “Forms” Authentication from user interface. Solution? PowerShell!
Add-PSSnapin Microsoft.SharePoint.PowerShell -ErrorAction SilentlyContinue $WebApp=get-spwebapplication "http://sharepoint.crescent.com" $WebApp.useclaimsauthentication="True" $WebApp.Update()