Install Drupal 8 on MAMP: can't detect version of Apache
Drupal uses Apache's ServerTokens in system_requirements() to determine the version number. You currently have that set to "Prod", which just reveals "Apache" as the token, without any version number.

For Drupal to detect the major and minor versions, plus revision, you need to update the config setting to one of:


Full - Server sends (e.g.): Server: Apache/2.4.2 (Unix) PHP/4.2.2 MyMod/1.2.
Min - Server sends (e.g.): Server: Apache/2.4.2.
OS - Server sends (e.g.): Server: Apache/2.4.2 (Unix).