I have a fresh install of CiviCRM (5.27) running on a Google Cloud Server Debian buster.
System is fully updated WordPress is 5.4.2
I wish to use Geocoding and of course noticed it is not in the searchable list of extensions.
I went to the download page and see that it says 5.26 - the documentation here leads me to believe that means "at least 5.26" so 5.27 should (in my mind - and correct me if I am wrong) be ok.
I put the zip file in the ext directory and changed the ownership to www-data:www-data and unzipped.
I refreshed extensions loader and saw it there and pressed install and then got a blank screen (civicrm themed so not a WP error) saying my wordpress install had a critical error.
Refreshing the page still shows the option to install and everything else works.
I hope I have given sufficient detail and am prepared to do any level of debugging.
I did not wish to try anything in case anyone wishes to see this error pristine (I did try install one second time).
PS, here is an error log entry:
[Tue Jul 07 00:57:43.003646 2020] [php7:error] [pid 26414] [client 18.104.22.168:56972] PHP Fatal error: require( ): Failed opening required '/var/www/html/civiwalk.com/wp-content/plugins/civicrm/civicrm/ext/org.wikimedia.geocode r-1.4/vendor/composer/../symfony/polyfill-mbstring/bootstrap.php' (include_path='.:/var/www/html/civiwalk.com/wp-co ntent/plugins/civicrm/civicrm:/var/www/html/civiwalk.com/wp-content/plugins/civicrm/civicrm/packages:/var/www/html/ civiwalk.com/wp-content/plugins/civicrm/civicrm/vendor/tecnickcom:/var/www/html/civiwalk.com/wp-content/plugins/civ icrm/civicrm/vendor/phpseclib/phpseclib/phpseclib:/var/www/html/civiwalk.com/wp-content/plugins/civicrm/civicrm/ven dor/pear/pear_exception:/var/www/html/civiwalk.com/wp-content/plugins/civicrm/civicrm/vendor/pear/auth_sasl:/var/ww w/html/civiwalk.com/wp-content/plugins/civicrm/civicrm/vendor/pear/log:/var/www/html/civiwalk.com/wp-content/plugin s/civicrm/civicrm/vendor/pear/console_getopt:/var/www/html/civiwalk.com/wp-content/plugins/civicrm/civicrm/vendor/p ear/pear-core-minimal/src:/var/www/html/civiwalk.com/wp-content/plugins/civicrm/civicrm/vendor/p in /var/www/html/c iviwalk.com/wp-content/plugins/civicrm/civicrm/ext/org.wikimedia.geocoder-1.4/vendor/composer/autoload_real.php on line 66, referer: http://www.civicafe.com/wp-admin/admin.php?page=CiviCRM&q=civicrm%2Fadmin%2Fextensions&action=add &id=org.wikimedia.geocoder&key=org.wikimedia.geocoder