Why am I getting unable to purchase message from App Store with free apps

Have you ever made purchases or downloaded content to this device before? If you set up an Apple ID and didn't want to use a credit card with the ID, then you set the ID up incorrectly.

Create iTunes Store account without credit card - Support - Apple

The Apple App Store requires a valid form of payment in order to download apps be they free or not.

Go to this site and sign in with your Apple ID and add a valid form of payment to your account.

https://appleid.apple.com/cgi-bin/WebObjects/MyAppleId.woa/

You will not be charged for free apps.

Sign out of your Apple ID, reboot yor iPad and sign in again.

Settings>iTunes and App Store. Tap your ID and sign out.

Reboot the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider if it appears on the screen - let go of the buttons. Let the iPad start up.

Go go back to Settings>iTunes & App Store. Sign in. Try to download the free apps again.