Activity->Progress : planned end date got a warning =>cannot update the activity

Hello,
I am trying to make my planning, and right now I am coping with the proper understanding of the progress zone.
For some reason, during my test, I got a warning in the field "planned end date".


On that screenshot I made my best efforts to put in some correct value.
1) What may be wrong with the dates I entered?

When trying to save my activity, I got the error message:


2 )I think that this is due to the "warning sign" in the field "planned end date".
Is that correct?

3) I understood that the "planned" dates are always calculated: no user can modify them directly.
Is my understanding correct?

4) If I want my Gantt chart to be showing the critical path I need to used the "fixed duration" or "regular between dates" mode.
Is my understanding correct?

Other wise I got the activity starting before the predecessor is complete, like here


5) Where can I find a user guide of "how to use that correctly?" for it appears more difficult than I thought.
Thank you.

Hello Kitcat,

you could check this out:
Problem with planned end date

Thank you #Chris,

Your question and the given answer on the link you provided enlighten my "n°1 bullet point" and I thank you for that.
Any Idea about my others points?