"Habla despacio, por favor."
Translation:Speak slowly, please.
Hablas is the tú form of the present tense (You speak, You are speaking)
Habla is the tú command form (Speak)
This should be ¡Hable despacio, por favor! since it is a polite command.
There's nothing in Duo's sentence that indicates we have to use a formal (polite) verb form.
You can use the word "please" with a tú (familiar, informal) command, an usted (polite, formal) command, or an ustedes (plural) command.
There is also nothing that says the polite form is incorrect. Hable and habla should both be accepted answers.
You incorrectly marked my answer wrong. "Hable" is correct for the Ud. imperative. The question does not specify whether tú or Ud. The reason given for marking it wrong also makes no sense on many levels--it is not using the imperative, and hable is only the yo form in the pres subjunctive, for which it also applies to the 3rd person!
Report it. Ranting about it on here isn't going to help the problem, as the team doesn't typically check the discussions. They DO check the reports.
I wasn't aware there was a mechanism for that. I thought I did it this way once before when I got a response from them.
Did you find the report button? For me it's at the lower left of the red "wrong" strip, next to the button for this discussion page. There's a "my answer should be accepted" choice. That will bring your answer to their attention.