I faced same error and below are my findings.
1:SDK installed version does not match with update you had on Ax,Means if your Ax has R3-CU8 install then open SDK version of CU8 which should be there along with default Retail SDK folder.
2:If you still unable to bypass this error go to reference folder in visual studio and right click on Microsoft.Dynamics.Retail.Pos.Contracts reference select properties and interop type to "false".