If the reference was broken after the SSIS package execution was scheduled, we may see an error similar to that shown below in the SQL Agent log for the job step that attempted to execute the SSIS package:
Failed to execute IS server package because of error 0x80131904. Server: vmSql16\Test, Package path: \SSISDB\Test\ParametersTest\SensitiveTest.dtsx, Environment reference Id: 35. Description: The environment ‘env2’ does not exist or you have not been granted the appropriate permissions to access it.
Andy has an explanation of what these are, how you might find them, and how to fix them.