SQL server bulk insert fail code 5 [SQLSTATE 42000] (Error 4861)
in-order to access a file in another machine using SQL Server you may give permission to the Account which you have used to start your sql-server instance.

if you have active directory on your environment make sure to start sql-server instance with an active directory user and give required permission to to that user on "Server-A". 
Change the Service Startup Account for SQL Server

but if you do not have any active directory on your Environment the only way to do this is to have a user with the exact name and password of your sql-server service user on Server-A