How to accept payment for digital goods in PayPal payment tool?

And, one more thing: as we are selling digital services so accepting payment for digital goods is necessary ?

You have to configure your PayPal account to accept payments of digitally delivered goods. I would suggest you to please enable it form your PayPal account. You may like to take a look at the following guide which should help you:

https://digitalgoods.co/2011/07/06/set-up-your-paypal-business-account-for-digital-goods/ 

https://developer.paypal.com/docs/classic/express-checkout/digital-goods/IntroducingExpressCheckoutDG/ 

Once you enable your PayPal account to accept payments of digitally delivered goods, you can configure your PayPal payment tool. Please check the screenshot below:

Hope this helps.

Do get back to us if you have any questions.