Multiple Brackets in Chemfig Polymer - Cannot Get Rid of Errors

From the documentation of chemfig, Section 13, the syntax @{<name>, <coeff>} can only be used at the beginning of the optional argument of a bond. In the second chemfig of your example

chemfig{[@{left,0.5}]C(-[2])(-[6])=C(-[2])(-[6])[@{right,0.5}]}

optional arguments [@{left,0.5}] and [@{right,0.5}] has no corresponding bonds, hence the nodes left and right are never created.

Option 1

If you are ok with beginning and ending bonds, then use

chemfig{C(-[2])(-[6])(-[@{left,0.5}4])=C(-[2])(-[6])-[@{right,0.5}]}
polymerdelim[delimiters={[]}, height=40pt, depth=, indice=n]{left}{right}

Option 2

If you want to keep the bond-less style, you can use @{<name>}<atom> to add a node to an atom, and use polymerdelim options open xshift and close xshift to adjust the position of delimiters. Note in this case, the left and right space is not auto-adjusted.

chemfig{@{left}C(-[2])(-[6])=@{right}C(-[2])(-[6])}
polymerdelim[delimiters={[]}, height=40pt, depth=, indice=n, 
  open xshift=-8pt, close xshift=]{left}{right}

Option 3

This is a variant of option 1, in which the beginning and ending bonds are specified but not drawn (by bond option [,,,draw=none]). To get better horizontal spacing, those two bonds are also shortened (by bond option [,0.5,,,]) and the position of nodes are adjusted (from @{left, 0.5}to@{left, 1}`).

chemfig{C(-[2])(-[6])(-[@{left,1}4,.5,,,draw=none])=C(-[2])(-[6])-[@{right,1},.5,,,draw=none]}
polymerdelim[delimiters={[]}, height=40pt, depth=, indice=n]{left}{right}

Full example:

documentclass[margin=20pt]{standalone}
usepackage{chemfig}

begin{document}

schemestart
  chemfig{R-C(-[2])(-[6])-C(-[2])(-[6])-C(-[2])(-[6])-lewis{0.,C}(-[2])(-[6])}
  +
  %% option 1
  % chemfig{C(-[2])(-[6])(-[@{left,0.5}4])=C(-[2])(-[6])-[@{right,0.5}]}
  % polymerdelim[delimiters={[]}, height=40pt, depth=, indice=n]{left}{right}
  %
  %% option 2
  % chemfig{@{left}C(-[2])(-[6])=@{right}C(-[2])(-[6])}
  % polymerdelim[delimiters={[]}, height=40pt, depth=, indice=n, 
  %   open xshift=-8pt, close xshift=]{left}{right}
  %% option 3
  chemfig{C(-[2])(-[6])(-[@{left,1}4,.5,,,draw=none])=C(-[2])(-[6])-[@{right,1},.5,,,draw=none]}
  polymerdelim[delimiters={[]}, height=40pt, depth=, indice=n]{left}{right}
  arrow
  chemfig{R-C(-[2])(-[6])-C(-[2])(-[6])-[@{left,0.5}]C(-[2])(-[6])-C(-[2])(-[6])-[@{right,0.5}]C(-[2])(-[6])-lewis{0.,C}(-[2])(-[6])}
  polymerdelim[delimiters ={[]}, height=40pt, depth=, indice=n]{left}{right}
schemestop

end{document}