While building webparts for office 365 or sharepoint online you need to develop sandboxed webparts.
To develop and deploy sandbox webparts you need to do few things for successful output.
- Download and install “Visual Studio SharePoint power tools”(http://visualstudiogallery.msdn.microsoft.com/8e602a8c-6714-4549-9e95-f3700344b0d9)
- Activate the service “SharePoint 2010 User Code Host” in services. So that error will go.
- For more info see at: http://sp2010solutions.blogspot.in/2012/03/sandboxed-solution-error-occurred-in.html
- In Visual StudioàNew ProjectàEmpty SharePoint Project