Can i use jelly powder or agar powder instead of gelatin to make whip cream?

I didn't think of it at the time- but you can also whip the liquid found in the cans of chickpeas. It's called aquafaba, and vegans use it as a replacement for egg whites and to make whipped cream. You can make your own using dry chickpeas, but that takes a bit of time. Basically, you cook the dry peas and then remove them from the cooking liquid. The liquid then gets cooked down a bit to thicken and remove excess water. Once that cools, it will whip. I've only done it once, using canned liquid. Tasted a bit beanish, plain, but was acceptable once sweetened and flavored. Not good enough to replace whipped cream for me, but I could see the potential uses otherwise. As I understand, the cooking liquid from other types of beans will also work- but I'm thinking black beans might give odd color to things.

Given you live in Indonesia, you should be able to find coconut cream easily. If you chill that well, it will whip much like whipped cream. You would have a coconut taste, but it shouldn't be objectionable. The problem with agar powder is it has to be heated just like gelatin, and combined with milk what you'd get is more a jelly than a cream. If you can find nondairy creamer, that also can be used to create a fake whipped cream. But if you ask me, I'd go with the whipped coconut cream for you cake. I'm not sure what the main thickener is in the jelly powder, but it is likely to be either gelatin or agar in an unflavored form. I would expect the end product to be more jelly like than whipped cream like. For the coconut cream option, use about 500ml of coconut cream. Get is as cold as you possibly can, plus get your whipping bowl and whisk as cold as possible as well. It will whip up much like cream, and you can sweeten it and flavor it to suit yourself.

You can use butter and milk if you don't have access to heavy cream,
https://www.leaf.tv/articles/substitutes-for-whipping-cream/

There are a number of methods/ingredients you can use to make whipped cream for this cake. And not have to use gelatin. You can't just whip gelatin into milk. You have to bloom it and heat (I know that you have to heat the agar agar) to dissolve then add it to the milk. Let us know if any of the substitutes work (or not).

I'm not familiar with the dish, but unless the whipped cream is structural, as in a parfait, I'd just skip it. If it is, I'd find another recipe.