I'm Trying to Dehydrate Marshmallows but they're chewy not crunchy

I suspect that in order to get a crisp texture, you would need to freeze dry the marshmallows with possibly dry ice or a rather expensive freeze dryer/food dehydrator. You can also try using your home freezer. There are instructions online.

You just need to have a little more patience and apply heat for a longer period of time. Your temperature is fine, the timing is too short. You need to give it at least 6 hours, and possibly as long as 12- 18 hours, depending on the relative humidity of where you are. You also need to figure out a way to slightly prop open the oven door during this time, so excess moisture has a way to escape. Also, use a spatula to flip the marshmallows over every 2 hours or so, so there is more even dehydration. I'd also recommend using a parchment paper underneath them. When they are going to feel firm, but not necessarily rock hard when first done. As they cool, that should happen. Also, don't leave them in the oven for so long afterwards. Marshmallows are still primarily sugar, which is naturally hygroscopic- that is, very attractive to water. Your dehydrated marshmallows will be thirsty creatures, and will work their hardest to reabsorb water from the air every chance they get. So as soon as they are cool enough to easily handle, get them packed in something airtight. That's also a good time to put in one of those silica gel packets, so any moisture in the packaging can get trapped. Good luck with your endeavor, and just be patient. Low heat and time will get you what you want.