Moodle checks the path given and if it can't use the path, doesn't enter into table mdl_config or mdl_config_plugins - web form shows an X not a check.
So now the question is what is @ fault ... Moodle or very restrictive Plesk environment where you host.
My vote ... the latter. Based upon helping a gentleman who was hosted with a provider that used Plesk.
From what I re-call, had difficulty with putting moodledata directory outside of code root ... which other providers (non-Plesk) allow. Finally hit upon the settings in 'open base dir' (check moodle's admin interface Server PHP info to see what that might be).
It's defined for every domain you run and to include a path, make sure you use the 'separator' character for multiple paths that Plesk requires.
Googling you found info ... where was that info? A Plesk forum? A forum for customers of hosting provider?
Can you actually test that path via Terminal session (ssh) by using either of the following commands:
/opt/plesk/php/<PHP version>/bin/php -m
/opt/plesk/php/<PHP version>/bin/php -i
And a biggy ... did you install your moodle manually or from some 'one click wonder' (Softac/other)?
And is tech support of where you host no help?
Am gonna venture a guess that of the users in this community forum not many host using Plesk (if any!).