documentclass{standalone}
RequirePackage{luatex85}
usepackage{fontspec}
usepackage{siunitx}
usepackage{datatool}
usepackage{filecontents}

begin{filecontents*}{myfile.csv}
filename,date,pressure
blabla,2018-03-12,66
end{filecontents*}

newcommand{getfirstrowfromcol}[2]{%
DTLsetseparator{,}%
DTLifdbexists{mydb}{DTLdeletedb{mydb}}{}%
DTLloadrawdb{mydb}{#1}%
DTLgetvalue{myname}{mydb}{1}{dtlcolumnindex{mydb}{#2}}%
myname%
}

begin{document}
SI{getfirstrowfromcol{myfile.csv}{pressure}}{bar}
end{document}

! Undefined control sequence.
GenericError  ...
                                                    #4  errhelp @err@     ...
l.18 DeclareSIUnit
                  A  {        ampere }

documentclass{article}
usepackage{siunitx}
usepackage{datatool}
usepackage{filecontents}

begin{filecontents*}{myfile.csv}
filename,date,pressure
blabla,2018-03-12,66
end{filecontents*}

newcommand{getfirstrowfromcol}[2]{%
DTLsetseparator{,}%
DTLifdbexists{mydb}{DTLdeletedb{mydb}}{}%
DTLloadrawdb{mydb}{#1}%
DTLgetvalue{myname}{mydb}{1}{dtlcolumnindex{mydb}{#2}}%
myname%
}

begin{document}
SI[parse-numbers=false]{getfirstrowfromcol{myfile.csv}{pressure}}{bar}

getfirstrowfromcol{myfile.csv}{pressure}

end{document}