[SOLVED] Fraction of days for Activity duration

Hello people.
We're just starting to use PorjeQtor for the first time and as a Newbee user, I have my first Newbee question. And I did try to find the answer in the forum before I asked.  
Here it goes:
I'm putting together a project and assigning tasks.
I get into trouble when I try to enter a fraction of day for duration (ex: 0.5d or 0.25d),  I get blocked and an exclamation point warning. (see attached screenshot)
Is there an obvious reason for this like a setting or other rule that I did not find in the manual?

Thanks for your help.  I will be back with more, I'm shure.
  
 
 

Thanks Arkhanje.
Can you suggest where I can find an explanation of Manual Planning or something that will help me understand the workflow concepts and options I have?  I'M searching the help files with no results.
I have also looked at many videos that don't cover an entire workflow. 
Thanks  
Martin

Correct me if I'm wrong.
I realize that there is a difference between PROGRESS-Duration, and Ressource Assigned Work.
From what I understand,
Progress Duration is for planning the timescale of the project (Only in full days if entered directly) ,
Ressourced assigned work which is the actual planned work and cost of the work.(Can be fraction of days.)
Ex:
If I plan 1 day for an activity in the Progress duration field, The gantt chart will display 1 day,
Once you assign work to a resource like 0.5 day, the cost will be calculated using 0.5 day and not 1 day.
Thanks for your help.