Can't change dates for task - Grey dates

What is your Task Constraint on the tasks?

Are you trying to change both the Start and Due on dates or only one?

If you are trying one of the dates, you might have to check the "schedule mode".

when the schedule mode is "Start Date" you will only be able to change the Start on date of the project, when its "Completion Date" you will only be able to change the Due on date.

If its not this case, Please check other project details or share a screenshot to understand further.

Please mark this reply as "Correct reply" if this helped you and to help others. TIA!

Your project has an issue where the timeline re-calculation has failed for some reason. Does it go back to normal if you hit 'Recalculate Timeline'? 

We have this happen a few times per year.  I understand this is a known issue and I have had 3-4 open cases with Adobe Service over the past 18 months.  Typically this can happen on a larger project and even on a larger project (think several thousand tasks, not several hundred, or a large amount of items in the Document Section - for us it was several hundred) should take only a few minutes to update.  This greying out is Workfront "thinking."  Doing a manual recalculation of the timeline has been the recent recommended fix and often helps, but in my mind, it's like CTRL+ALT+DEL and clear/cache cookies response.  I have found an improvement in our instance, but for a while, the dates would grey for 4-5 hours at a time, making all progress creating or adjusting your plan come to a screeching halt.  We are not on Fusion, Workfront Classic.