At `tabularray` in contrary to other tables' packages in each rows regardless if in them are multicolumn cells (written 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 $n_{\ce{Cl}}$} 
 & & \SetCell[c=2]{c} {Chlorination degree of \\ 
 olefinic compounds $n_{\ce{Cl}}$}
 & \\
 & 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}
```

[![enter image description here][1]][1]

**Edit:**
For chemical elements (and formulae) is sensible to employ `mhchem` package and write symbol for chlorine `\ce{Cl}` or n-clorine `$n_{\ce{Cl}}$`. 


**Off-topic:** Your table is wider than text area width. So, you may consider the following table preamble:
```
\begin{tblr}{colspec={l *{4}{X[c]}}}
```
which gives:

[![enter image description here][2]][2]



 [1]: https://i.sstatic.net/FltR9.png
 [2]: https://i.sstatic.net/dfJoT.png
 [3]: https://i.sstatic.net/FaD6C.png