Why coffee tastes bitter
Harsh bitterness often comes from over-extraction, too-fine grind, or water that is too hot.
Some bitterness is normal. Harsh unbalanced bitterness is often over-extraction, too-fine grind, or water that is too hot.
Fixes
Grind coarser or shorten contact time. Let boiling water sit briefly. Ease a very high dose. Try a lighter roast for filter if the cup is only roast-bitter.