Look at the following discussion,
Below is excerpt from above thread.
The relationship endpoint instance does not exist. : Rel(clsId:BMC.CORE:BMC_DEPENDENCY instId: AG00215AF33B3889IWSgrKxpAAYhQB can't be created. The supplied L-hand datasetId doesn't match datasetId of endpoint inst. Supplied: BMC.ASSET L-hand instId: OI-7F2CF3EF52F14F7BBA7F86627223D22C datasetId: BMC.HWSW.S (ARERR 120040)
would come when any of the Source CI or Destination CI Recon ID is null or incorrect... so before the recon id is generated if you try to push a record in dependency form to make a relation between these '2' CIs you should experience this error...