Enable OAuth using Powershell – run as administrator: (Shift and Right click)

Add-PSSnapin Microsoft.Crm.PowerShell

Check the settings:

Get-CrmSetting -SettingType OAuthClaimsSettings

$ClaimsSettings = Get-CrmSetting -SettingType OAuthClaimsSettings

$ClaimsSettings.Enabled = $true

Set-CrmSetting -Setting $ClaimsSettings