Performed test 1 as you recommended in the referenced post. Same results as before. There is no output from the install script. All that happens is the admin home page comes up after clicking "run script". No change on second run of script. No difference when running the check files script. The SBA menu item is not placed in the admin/catalog menu.

I installed a clean (unmodified) zc version 1.5.7c, installing the test data. I loaded all the SBA files in the March 18 version from Github that were in the admin and includes folders and the files in the 1.5.7 folder. The YOUR TEMPLATE files were placed in the template_default folder and the classic green template selected. I did not install the Dutch or French language files. No merging of the five files that were different from the base zc files was done; the SBA files were written over the 1.5.7c files.

I logged in to admin and added stock_by_attr_istall.php to the url and hit return. The help/select script page comes up as expected (Stock By Attribute (SBA) installation script SBA Version 1.5.4 for Zen Cart Version 1.5.7c). I selected the Full/Upgrade DB install script and clicked run script.

The url in the browser admin home page url window following running the script was my_domain/admin/index.php?cmd=home&selectSBAinstall=installAll&getSBAinstallPage=Run+Script if that helps.

I'm running with Mac OS 10.15.7 (Catalina) with Firefox (86.0.1) on a local development MAMP Pro 6.0.1 server running php 7.3.21, Apache 2.4.46, and MySql 5.7.30.

Hope this helps. What configuration are you using that won't reproduce the problem?

Dave