Hi Manasa,
you can change it to error message using the DBM customizing: (/DBM/SPRO) Dealer Business Management->Order->Basic Functions->Define Schema for Message Substitution.
Or in DBM 6.0 (/DBM/SPRO) Dealer Business Management->Order->Order Control->Define Schema for Message Substitution.
here you enter the message WTY15 008 and change the type to error.
However also you have to implement the sap note 1364421 - Warranty Claim can be created without partner
Regards
Ganesh