I believe you have some misunderstanding of subdomain and subfolder (or subdirectory).
a primary domain abc.com
a subDOMAIN would be 123.abc.com the 123. in front of the primary domain
A subfolder would be abc.com/123 the 123 at the end of the primary domain.
you can also have a subfolder in a subdomain 123.abc.com/store
If you have an installation in a subdirectory abc.com/store
Edit file .htaccess If abantecart is installed in yourdomain/ subfolder change configuration line
RewriteBase / to RewriteBase /subfolder/. # Example: yourdomain.com/store use RewriteBase /store/ # Important to have / at the end of the line.
and rename your .htaccess.txt file to .htaccess - note the . (dot) at the beginning must remain.