I am trying to make a tetris game, but I don't understand this error? It seems to be line 34:

self.active_blk.move(-direction)

Here is my code:

import pygame
import random
from Block import Block

class Stage():
def __init__(self,cell_size,h_cells,v_cells):
    self.cell_size=cell_size
    self.width=h_cells
    self.height=v_cells
    self.blocks=[]
    self.active_blk=self.add_block()

def add_block(self):
    blk=Block(0,self.cell_size,(random.randint(0,255),random.randint(0,255),random.randint(0,255)))
    self.blocks.append(blk)
    return blk

def move_block(self,direction):
    self.active_blk.move(direction)

    obstacle=False
    for cell in self.active_blk.cells:
        if(cell.y>=self.height or 
           cell.x<0 or 
           cell.x>= self.width): obstacle=True

    for blk in self.blocks:
        if(blk is self.active_blk): continue
        if(blk.collide_with(self.active_blk)):
            obstacle=True
            break;

    if(obstacle):
        self.active_blk.move(-direction)

def draw(self,screen):
    screen.fill((0,0,0))  
    for blk in self.blocks:
        blk.draw(screen)