Also one more thing I noticed, when I change POST request to get request, I am getting same error.

Expression evaluation failed. The message body is not a valid JSON. Unexpected character encountered while parsing value: <. Path '', line 0, position 0. at Newtonsoft.Json.JsonTextReader.ParseValue() at Newtonsoft.Json.Linq.JObject.Load(JsonReader reader, JsonLoadSettings settings) at Microsoft.WindowsAzure.ApiManagement.Proxy.Gateway.MessageBody.AsJObject(Stream stream, Encoding encoding, JsonSerializerSettings settings) at Microsoft.WindowsAzure.ApiManagement.Proxy.Gateway.MessageBody.As[T](Boolean preserveContent)

I am getting error at below line.

    <set-header name="Authorization" exists-action="override">
        <value>@("Bearer " + (String)((IResponse)context.Variables["bearerToken"]).Body.As<JObject>()["access_token"])</value>
    </set-header>

Here is my complete Policy

<inbound>
<base />
<set-variable name="token" value="@(context.Request.Headers.GetValueOrDefault("Authorization","scheme param").Split(' ').Last())" />

    <set-header name="Authorization" exists-action="delete" />
    <!-- Send request to Token Server to validate token (see RFC 7662) -->
    <send-request ignore-error="true" timeout="20" response-variable-name="bearerToken" mode="new">
        <set-url>https://login.microsoftonline.com/{tanentid}/oauth2/v2.0/token</set-url>
        <set-method>GET</set-method>
        <set-header name="Content-Type" exists-action="override">
            <value>application/x-www-form-urlencoded</value>
        </set-header>
        <set-body>@{
            return "client_id={xxx}&scope={api://xxxx/.default}&client_secret={xxxx}&grant_type=client_credentials";
            }</set-body>
    </send-request>
    <set-header name="Authorization" exists-action="override">
        <value>@("Bearer " + (String)((IResponse)context.Variables["bearerToken"]).Body.As<JObject>()["access_token"])</value>
    </set-header>
    <set-backend-service base-url="https://nma3-dpioto-bld-fa2.azurewebsites.net/api/bus/inbound-general-q" />
</inbound>

Thanks @JananiRamesh-MSFT . I used your script to send request for generating bearer token, however I am getting below error. Also I think to note, when I use POSTMAN client to get Bearer token, I get it with GET request in POSTMAN not POST.

Error Details : 401 Unauthorized. Similar to 403 Forbidden, but specifically for use when authentication is possible but has failed or not yet been provided. The response must include a WWW-Authenticate header field containing a challenge applicable to the requested resource

Bearer realm="nma3-dpioto-bld-fa2.azurewebsites.net" authorization_uri="https://login.windows.net/{tanentid]/oauth2/v2.0/authorize" resource_id="xxxxx"