No worries - You definitely did NOT mess up.
You don't "have" to do a Liver Flush before you start.
How long an Master-Cleanse are you doing? My understanding is, the longer the MC, the more recommended the liver flush.
However, flushing is up to you. If you really don't want to do one, and don't feel you have any issues that could benefit from a liver flush, don't do one.
You are the best judge of your body and what it needs.