"I have to tell you something right away."
Translation:Je dois te raconter quelque chose tout de suite.
It does accept that form; I just used it. But you need to write "je dise" (le subjonctif) not "je dis" (l'indicatif). "Il faut que" always requires the subjunctive mood. Note that this course barely begins, in its most advanced lessons, to teach the subjunctive, but I was pleased and surprised that it took it here.