SAP Message 06510 - According to quota arrangement, it is not this source's turn

Diagnosis
The material you want to order is subject to aquota arrangement. According to the quotaarrangement computation, the material should be procured from thefollowing sources of supply:
procurement type &V1&
special proc. type &V2&
vendor &V3&
procurement plant &V4&

Procedure
Determine whether the material is in fact to be procured from theselected source.