Because you have a Suburban Water Heater (from your description)
Need a 1 1/16" SIX point socket to remove your Anode Rod (drain plug)
Try to slightly 'tighten' FIRST then unscrew it.
Might need the help of a cheater bar.
After you get it out......
Use a 3/4" steel nipple to clean up/chase the drain hole threads
Use a hack saw to cut three slots in threads of steel nipple....lengthwise
They will aid in cleaning up the water heater tank drain hole threads
Use a couple wraps of Teflon Tape on new anode rod threads when you install it.