正規表現で [ と ] をエスケープする必要があります (Substitute/Substitute Into を使用することもできます)
Names Default To Here(1);
txt = "a],[b";
t1 = Regex(txt, "\!],\!\[", "\!n", GLOBALREPLACE);
t2 = Substitute(txt, "],[", "\!N");
show(t1, t2);
https://www.jmp.com/support/help/en/17.0/index.shtml#page/jmp/jsl-syntax-rules.shtml
https://www.jmp.com/support/help/en/17.0/index.shtml#page/jmp/escaped-characters-in-regular-expressions.shtml
],[
ありがとう!
t1=Regex(txt,"],[","\!n",globalreplace);//??