have this error when Compile for relay switch! :(

Just go to the Arduino site.
Hit the "Download" section.
Select "PREVIOUS RELEASES"

Don't go below 1.6.4 and don't go above 1.6.5.
Currently running 1.6.5 with no issues at all here.

See this post for a bit more info.

iot-playground.com/forum/my-project/497-error-in-ide#2615
Its quite important to remove the older libraries or you may end up with similar issues.