With an average depth of about 12,000 feet, the extremes of darkness, high pressure, and cold that characterize the deep sea could not be tolerated by shallow-water animals.
I do not understand why this sentence is incorrect,,,

It’s a dangling modifier. What has an average depth of 12,000 feet? Not the extremes of darkness. The DEEP SEA has that average depth, so that’s what needs to come after the first comma.