with or without minä should be both correct. it is annoying that sometimes the system says the sentence is incorrect when you use the minä/sinä/me/te in the sentence, and on other occasions it is the other way around. Please fix this asap.
Tyttö is the subject of the verb on, so it should be nominative. The object of ajattelen is jota, which needs to be partitive. While jota refers back to tyttö, it doesn't need to be in the same case as tyttö.