That is not the correct PTO clutch for your mower and some one has done a "Hillbilly" modification to one they had to sort of make it work or fitted a PTO to a mower that did not have one.
It would have been really nice if you had posted your model & serial numbers but such is life.
According to the ARI, MTD made 5 variations of the Yardman Lawn tractor in 1998 and none of them had an electric PTO.
The Garden tractors did have an electric PTO and there were two variations
THis is where those numbers would really have helped those who are trying to help you but I don't want to sound like I am nagging, even though I am actually nagging.
View attachment 25995
This appears to be what you have and the clutch ( No 110 ) is secured by the plate ( No 112 )
Or it could be this set up
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Or in this example where the clutch ( No 97 ) is secured by a different plate (No 96 )
In both these cases the plate has a free end that goes into the slot .