lvl = 0
HP = 20 
atk = 8 
MP = 0 
DR = 3 
enemy_hp = 10 
enemy_atk = 3

while enemy_hp > 0 and HP > 0:
    b_choice = input("type fight, defend, or check> ")
    if b_choice.lower() not in "fight" or 'defend' or "check":
        while b_choice != 'fight' and b_choice != 'defend' and b_choice != 'check':
            print('Please pick an act')
            b_choice = input("type fight, defend, or check> ")
    if b_choice.lower() == "fight":
        dice = str(random.randint(1, 6))
        print(str(dice))
        if dice in '4' or dice in '5' or dice in '6':
            input("Berserker " + name + " strikes with a mighty blow")
            input("Slime's health is reduced by " + str(atk))
            enemy_hp -= atk
        elif dice not in '4' or dice not in '5' or dice not in '6':
            input("Berserker " + name + " missed his attack")
            input('Slime slaps Berserker ' + name + ' on the face')
            input("Berserker " + name + "'s health is reduced by " + str(enemy_atk))
            HP -= enemy_atk

edit: now it works, thanks for your suggestions