Could you please explain? What is ungrammatical about "...as soon as I finish the preparations"?
According to me both are grammatical. However, since the words "as soon as" were used they instinctly let me use "have finished" instead of "finish".
Don't you share the same?
I would probably also use "have finished" but only because I'm a stickler.
Many, many people would say:
"I will do my homework as soon as I finish my dinner"
"I will brush my teeth as soon as I put on my pyjamas"
"I will wash the floor as soon as I finish the ironing"
I don't believe that "as soon as" would play a role in the choice.
In casual conversation, I think this would be the more common construction.