The answer to 'water heater installation won't flush' is that a clogged or blocked drain line is the most common cause. This can happen if sediment or debris builds up in the drain line, preventing the water from draining properly during a flush. Symptoms include the water heater not fully draining or taking a long time to drain.
To resolve this, you'll need to clear the drain line. This may involve using a plumbing snake or auger to remove the blockage. If the blockage is severe or difficult to access, it's best to call a professional plumber who has the right tools and experience to properly clear the drain line.