At tabularray in contrary to other tables' packages in each rows regardless if in them are multicolumn cells (write as \SetCell[c=2]{c} ... ) had to be all ampersands as defined in table preamble. In your case, the MWE should be:
\documentclass{article} \usepackage{rotating} \usepackage{makecell} \usepackage{tabularray} \usepackage[version=4]{mhchem} % <--- new \begin{document} \begin{table} \footnotesize \begin{tblr}{colspec={c *{4}{Q[c,3cm]}}} & \SetCell[c=2]{c} {Chlorination degree of \\ paraffinic compounds \ce{Cl}} & & \SetCell[c=2]{c} {Chlorination degree of \\ olefinic compounds \ce{Cl}} & \\ & non-exposed (0h) & exposed (144h) & non-exposed (0h) & exposed (144h) \\ Non-hydroxylated & $8.80\pm 0.00$ & $9.07\pm 0.00$ & $8.37\pm 0.02$ & $8.55\pm 0.01$ \\ Mono-hydroxylated & $8.31\pm 0.06$ a) & $7.59\pm 0.01$ & $8.31\pm 0.06$ a) & $7.04\pm 0.03$ \\ Di-hydroxylated & n.d. & $6.61\pm 0.04$ & n.d. & $5.23\pm 0.01$ \\ \end{tblr} \end{table} \end{document} 
Edit: For chemical elements (and formulae) is sensible to employ mhchem package and write symbol for chlorine \ce{Cl}.
Off-topic: Your table is wider than text area width. So, you may consider the following table preamble:
\begin{tblr}{colspec={c *{4}{X[c]}}} which gives:
