Since you have already checked to see if the sprite exists, you don't actually need to check again. With the Else added, you know the sprite exists at the point the error was occuring.
Also, a minor point (only mentioned because I know you are currently into optimizing) is that if the sprite is being deleted, then the previous positioning of the sprite is a wasted exercise.
So, moving it to the Else section only positions the sprite if it hasn't been deleted (when either l1burtshp(n) or the sprite's X position becomes less than zero).
_level_1:
counter=counter+1
for n=100 to 200
if sprite exist(n)=0
create animated sprite n,"enemies/brotoburt.bmp",1,4,10
sprite n,590,rnd(430)+50,10
play sprite n,1,4,20
Else
if sprite visible(n)
if l1burtshp(n)<=0 or sprite x(n)<0
delete sprite n
Else
sprite n,sprite x(n)-2,sprite y(n),10
endif
endif
endif
next n
`this part defines when enemies are shown
if counter=100 then show sprite 100
return
TDK_Man