You get this error in SDTT when you create a date item for the social media profiles instead of just adding the link. To fix this error follow these steps.

Step 1: Delete the data items for the social media profiles. Go to your Data Items list, select the social media profile data item, click Actions>Delete.

Step 2: Go to your homepage schema markup. For SameAs, just paste the URL for each of them. Use the + sign to fix it.