SQL Server Raise Error (raiserror)

raiserror
(
  N'There is already an active Modification record for Servicer Loan Number: %s, you must close that modification before starting a new one.',
  16,-- Severity.
  1,-- State.
  @ServicerLoanNumber -- substitution argument.
)

NOTE: Use %s for string substitution, %n for number substitution, etc.