The following data source elements cannot be mapped to custom Office properties because their data types are incompatible: ...

The following data source elements cannot be mapped to custom Office properties because their data types are incompatible: {0}. If there are Office properties corresponding to these data source elements, map them. Otherwise, the fields corresponding to these data source elements will not be shown on Outlook forms. You may set default values on those fields in create operations of this external content type to ensure that proper defaults will be set on them while creating new items.