The WS Federation protocol uses STS. Therefore, you must enable the attribute set for STS to use it in an WS Federation relationship.
Click Devices > Identity Servers > Servers > Edit > WS Federation > STS Attribute Sets.
Move the WS Federation attribute set to the Attribute sets list.
Select the WS Federation attribute set and use the up-arrow to make it first in Attribute set.
Click OK.
Update Identity Server.