How to arrange color box vertically parallel to each other in a color box?

There is no need for tcbraster in this case. The problem comes from the empty lines between boxes and hfill. An empty line in LaTeX means to start a newparagraph, therefore nobeforeafter option and hfill have no effect. Just delete these empty lines or comment out, you get boxes A and B one beside the other.

documentclass[border=10pt]{standalone}
usepackage[most]{tcolorbox}

begin{document}

newtcolorbox{GreenBox}[2][]{%
   enhanced,
   colback   = green!5!white,
   colframe  = green!75!black, 
   arc       = 4mm, 
   outer arc = 1mm, 
   fonttitle = Hugeslshapetextbf,
   center title, 
   title     = #2,
   #1}

newtcolorbox{BlueBox}[2][]{%
   enhanced,
   colback   = blue!5!white,
   colframe  = blue!65!black, 
   arc       = 4mm, 
   outer arc = 1mm, 
   fonttitle = Largeslshapetextbf,
   center title, 
   title     = #2,
   #1}
 

begin{GreenBox}[width=11cm, remember as=box1, nobeforeafter]{ABC}

  begin{BlueBox}[width=5cm, remember as=box11, nobeforeafter]{A}
  Some Text here.
  end{BlueBox}
  %
  hfill
  %
  begin{BlueBox}[width=5cm, remember as=box12, nobeforeafter]{B}
  end{BlueBox}
    
  begin{BlueBox}[width=5cm, remember as=box13, nobeforeafter]{C}
  end{BlueBox}
end{GreenBox}


end{document}

Update: After my answer I've seen that OP wants box C beside box B and I did it wrong. The best solution could be to declare box C just after box A and box B after them. Another solution could be to use a multi column environment inside the tcolorbox.

I have tried the following codes. But I have an issue the width of the boxes.

documentclass[border=10pt]{standalone}

usepackage{enumitem}
setlist[itemize]{leftmargin=*, itemsep = 0em}
usepackage[none]{hyphenat}

usepackage{tikz}
usetikzlibrary{matrix, shapes, arrows, positioning}

usepackage[most]{tcolorbox}


begin{document}

newtcolorbox{GreenBox}[2][]{%
   enhanced,
   colback   = green!5!white,
   colframe  = green!75!black, 
   arc       = 4mm, 
   outer arc = 1mm, 
   fonttitle = Hugeslshapetextbf,
   center title, 
   title     = #2,
   #1}

newtcolorbox{BlueBox}[2][]{%
   enhanced,
   colback   = blue!5!white,
   colframe  = blue!65!black, 
   arc       = 4mm, 
   outer arc = 1mm, 
   fonttitle = Largeslshapetextbf,
   center title, 
   title     = #2,
   #1}

 begin{tcbraster}[raster columns=3, raster valign=center] 

begin{GreenBox}[width=10cm, remember as=box1, nobeforeafter]{Objectives 1}

 begin{tcbraster}[raster columns=1, raster valign=center] 
  
  begin{BlueBox}[width=10cm, remember as=box14, nobeforeafter]{Objective 1.1}
  Some text here.
    end{BlueBox}
  end{tcbraster}
  end{GreenBox}
  begin{GreenBox}[width=5cm, remember as=box2, nobeforeafter]{Objective 2}
Some text here.
end{GreenBox}
  begin{GreenBox}[width=5cm, remember as=box3, nobeforeafter]{Objective 3}
Some text here
end{GreenBox}
end{tcbraster}

end{document}