Incomplete installation, probably an http/https ajax issue

Hello,
After installation I compiled the “Resume Installation” Popup but it seems doesn’t activate the plugin.
It continuosly pops up the Resume installation alert and all options are blocked.
Chrome and Firefox console show the error “Mixed Content: the page at http:mydomain/wp-admin/admin-ajax.php was loaded over https, but there request etc…”
Can I install in a different way?

Thank you
Paolo