Resource Credits are awarded to you everyday if you have Steempower. Normally if you have 15 Steem Powered up as Steem Power you should be able to perform most common transactions. If you have 15 Steempower and you are still seeing this messsge then you have used all your resource credits and you will need to wait 24 hours for them to recharge. If you go to the Community Steemit Explained you will find a post about resource credits.

https://steemit.com/hive-123507/@shortsegments/4qrz8q-what-are-resource-credits