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, find the thing data items, open them and click "delete"

Step 2: Go to your homepage schema markup. For SameAs, just paste the URL for each of the social media links (do not click "convert to data item").

Note: Use the + sign to add more fields for additional profile links.