(alltrim(table1.f1)) is the Name Expression and it's properly used in place of Popup Name. Use of (alltrim(table2.command1)) is invalis because FoxPro doesn't expect a name but a command. Only macro substitution is appropriate here.

lcOnSelCommand = alltrim(table2.command1)
on selection bar m.ix of (alltrim(table1.f1)) &lcOnSelCommand

IOW, the Name Expression can only be used where FoxPro expects a name.

>
>I want to achieve the following.
>
>

>For m.ix = 1 to ..
> define bar m.ix of (alltrim(table1.f1)) prompt (alltrim(table2.f1))
> on selection bar m.ix of (alltrim(table1.f1)) (alltrim(table2.command1))
>endfor
>

>
>The line "on selection bar m.ix of (alltrim(table1.f1)) (alltrim(table2.command1))" fails.
>If i remove the "(alltrim(table2.command1))" it works but it will be of no use to me.
>(alltrim(table2.command1)) contains valid foxpro command like do abc.prg
>
>but this works
>on selection bar m.ix of (alltrim(table1.f1)) do abc.prg
>
>error returned unrecognised command verb .
>I tried macro substitution like
>

>m.macro1 = "on selection bar"+alltrim(str(m.ix))+" of "+alltrim(table1.f1)+alltrim(table2.command1)
>¯o1
>

>and it worked
>
>Can someone highlight why didnt this work ??
>
>on selection bar m.ix of (alltrim(table1.f1)) (alltrim(table2.command1))
>