Communication Error ( Vertex ) in SAP SD
While entering the address details of sales organisation in defining the error appears as "Communication error with external tax system(Vertex_MS0018)"
VERTX is used for pricing and taxation purpose. Vertex is used by SAP as its own tax technology provider. Vertex is on a different system and you need a communication setup between sap and vertex.
If you don't have a vertex which can be linked to your IDES, then you need to have the vertex disabled in your IDES to proceed. Also, check if you have the Vertex Tax Calculation procedure attached to your country.
The problem of communication also arises when the tax jurisdiction code is not filled in the G/L account line and the tax line in the case of a US export business because the tax destination partner (ship-to party) is located in a country where no tax jurisdiction code is used. However, the FI/CO interface of the R/3 System always requires a filled tax jurisdiction code. Therefore, define the corresponding value in field Tax Jurisdiction Code ' in R/3 table T001.
Read more about SAP SD