Where is the "plugin.zip' file name coming from?
https://moodle.org/plugins/index.php?q=adaptable
https://moodle.org/plugins/download.php/16774/theme_adaptable_moodle35_2018051700.zip
The above link is what one would be downloading ... and that file would also be the one you drag and drop or upload by some other means to moodlecode/theme/
'spirit of sharing', Ken