SEO does NOT work despite adding .htaccess: 500 Internal Server Error!

My store is in a subfolder, so I put in .htaccess: /vexcor and it just won't work (I tried other variations, including the full server path, etc. as well).  It keeps giving a 500 Internal Server Error.

I'm using PHP 7.3.27 and FastCGI on Ubunto 16.04 and Apache 2.  It works great otherwise, but I can't get SEO links to work. Help please!