Multirow and multicolumn tabular for (one and a) half line

The use of multicolumn{3}{l}{} overwrites the vertical lines resulting in the small gap you see. Add the line back in using multicolumn{3}{l|}{}.

documentclass{scrarticle}
usepackage{multirow}

begin{document}
    
    begin{tabular}{|l|c|c|c|c|c|}
        hline
        multirow{3}{*}{No} & 
        multirow{3}{2.4cm}{Code} &
        multicolumn{3}{|c|}{multirow{2}{*}{Generated by }} &
        multirow{3}{2.5cm}{Generated and described on Layer} 
        
        & & multicolumn{3}{l|}{} &   cline{3-5}
        & & Master & Slave & Gateway & 
        hlinehline
        a & b & c & d & e & f  hline
    end{tabular}

end{document}

---

Update regarding the second part of the question: In the following example, I have used a nested tabular for the "Generated by" part of the header. With this approach, I got rid of all multirow commands. In order to make sure, teh last column's header takes up three rows, I used makecell from the makecell package, but you could also use a fixed width column type here. In order to make sure, the line below "Generated by" ends up higher, I used the cellspace package, the neccessary amount of the top and bottom limit were detemined via trial and error. Lastly, I used wc type columns from the array package in combination with widthof from the calc package to make sure columns 3 to 5 have the same width as their corresponding headers:

documentclass{scrarticle}
usepackage{array}
usepackage{calc}
usepackage{makecell}
usepackage[column=0]{cellspace}
setlength{cellspacetoplimit}{6pt}
setlength{cellspacebottomlimit}{cellspacetoplimit}


begin{document}
    
    begin{tabular}{|l|c|wc{widthof{Master}}|wc{widthof{Slave}}|wc{widthof{Gateway}}|c|}
        hline
        No & 
        Code &
        multicolumn{3}{@{}c@{}|}{begin{tabular}{0c|0c|0c} multicolumn{3}{0c}{Generated by}  hline Master & Slave & Gateway end{tabular} } &
        makecell[cl]{Generated and described on Layer} 
        hlinehline
        a & b & c & d & e & f  hline
    end{tabular}

end{document}