Ive encountered the following error, any idea what is causing this?

A wall has been placed at portion(s) of an OUTLET boundary condition (at 74 % of the faces, 74 % of the area)to prevent fluid from flowing into the domain. The boundary condition name is: outlet. The fluid name is: Water at 25 C.If this situation persists, consider switching to an Opening type boundary condition instead.

This was caused by recirculation at outlet. If this hapens at begining it can fade away as solution converging. If no, use opening BC or move outlet further downstream. For openning BC you need to specify temperature which you can obtain by using expression (see tutorial CHT in heating coil for details).