I want to believe that you made a mistake by using "Reply to author" instead of "Reply to group", but when you ask a public question, the answers should also be public.
You should - anyway - read the Inno Setup documentation about signtool!
Regarding your question, it is not Inno Setup that looks for where the signtool.exe executable could be, it is you who must tell Inno Setup where this executable is.
First of all, to do this, you have to install the Windows SDK:
After, using InnoSettup Tool Configure Sign Tool, you create an Inno Signtool "Standard" as I wrote previously :
Standard=$qC:\Program Files (x86)\Windows Kits\10\bin\10.0.22000.0\x64\signtool.exe$q $p $f
To know what $q, $p and $f correspond to read the documentation.
And only after doing all this, you can set the Signtool= line in the [Setup] section.
SignTool=Standard sign /fd sha256 /tr
http://ts.ssl.com /td sha256 /f $qC:\pathto\mysignfile.pfx$q /p mypassword $f
http://ts.ssl.com is the real time server you want to use