seem to cant find the boneID of the players back for setparent();     entity.SetParent(player, 3, true)

Check BaseEntity.GetModel().boneNames.

Im trying to set the entity parent to players back.  player.GetModel()   is null

when i do entity.SetParent(player, 3, true)

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StringPool.GetString - no string for ID3

Weird.  I used .GetModel().boneNames on the BradleyAPC and it worked.  (Never tried on a BasePlayer.)

Anyway, the backpacks plugin uses "spine3". That might be suitable for your purposes.