Thank you for reporting this issue. We believe that you can see this error because you have .json file open in your editor inside Mbed Studio. In version 0.7.0 of Mbed Studio we support intellisense (autocompletion, finding references, navigation, etc …) only for C and C++.
We are considering adding intellisense support for other languages like json, python but not exact timeframe is set for it yet.
Less advanced features like viewing, editing, saving files with .json extension should be fully supported.
Arek - Mbed Studio team