exchange

If KeyDown (72)

Player_y=Player_y-4

If Player_y<0 Then Player_y=0

EndIf


with


If KeyDown (72)

Player_y=Player_y-4
EndIf

If Player_y<0 Then Player_y=0

EndIf

Same with

If KeyDown (80)
Player_y=Player_y+4
If Player_y>400 Then Player_y=400
EndIf


Need to add endIfs although Im guessing you were trying to add different conditions. Try just having one if statement which is

If KeyDown (72) Then
player_y= player_y - 4
ElseIf KeyDown (80) Then
player_y= player_y + 4
EndIf

;this will be the one which moves the player up and down. and then there will be a seperate if statement to make sure you dont go off the screen

If Player_y>400 Then Player_y=400 then
ElseIf Player_y<0 Then
Player_y=0
EndIf


;its doesn't matter in blitz but if you add then after you check conditions its good programming habit.

youve gotta understand Im writing this help in a new window whilst i scroll down your post in the other and as Im scrolling down I notice how all your If statements are screwed up.

Youve got the right idea but rewrite all of them and make sure ever if has an endif. if you want to do another thing then add a elseIf. It should work fine then.
e.g

If Ball_x<1 Then p_score=p_score+10

If Ball_x>600 Then o_score=o_score+10

should be changed with

If Ball_x<1 Then
p_score=p_score+10
ElseIf Ball_x>600 Then
o_score=o_score+10
EndIf



hope you know what Im rambling about