This is a version compatibility problem. In your code you have mentioned to use version 0.4.15 but the compiler version installed in your system is 0.5.0 (mentioned in the error). There are two solutions to this.
- Use the current version pragma solidity 0.5.0;
- Use pragma solidity ^0.4.15;
pragma solidity 0.4.15; is mentioned to use the exact version whereas pragma solidity ^0.4.15; is mentioned to use the mentioned version and the above versions