Entity Framework SaveChanges fails due to a trigger
Most likely there is a problem with the JOIN to sometable that is filtering out any rows from matching, hence the @biInbndEquip_ID variable never gets populated.

HOWEVER, even if you fix that, you still have a bigger problem to deal with: your trigger logic is written to handle a single row. If a multi-row INSERT is done, then the trigger will only ever grab one of the values that was inserted to place into the Message214Status table. Get rid of the @biInbndEquip_ID, @iCust_ID, etc local variables in the trigger, and rewrite it to be a simple INSERT...SELECT. For example:

INSERT INTO dbo.Message214Status (InbndEquip_ID, Cust_ID, ...)
  SELECT ins.[InboundEquipmentID], ins.[CustID], ...
  FROM   INSERTED ins
  INNER JOIN sometable c
          ON c.[join_column] = ins.[join_column];