Placing the_content inside shortcode not working

It doesn’t appear that the variable $the_content exists. I’m guessing what you want is the function the_content(), but since that actually echoes its output, that won’t work either. You need to use the function get_the_content() which will return the output (as opposed to echoing it).

Try this:

echo do_shortcode('[wcm_restrict plans="silver"]' . get_the_content() . '[/wcm_restrict]');