There are some new rigify part types that have not been added to Uefy 2 just yet. Specifically limbs.front_paw and limbs.rear_paw. The default horse rig has been updated to use these types but they are not in Uefy 2 just yet. (Will be in future update)
You can fix this issue for the time being by changing them to limbs.super_limb as below.
Go in POSE mode and select thigh.L in Bone Properties → Rigify Type → Rig Type change it from limbs.rear_paw to limbs.super_limb then set sub option Limb type to ‘Paw’.
Similarly change the upper_arm.L from limbs.front_paw to limbs.super_limb with sub option ‘Paw’. Do the same on the other side too and that will fix your issue.
You can check the FAQ on the product page " What metarig sample types are supported?" to see list of currently supported rig types.
EDIT: You can set the abdomen and hip bone to basic.super_copy