i cannot install any plugin or delete it

hello
i get a critical issue,
i cannot install or delete or update any plugin
when i tried to install the plugin its install normally then when i try to activate it’s giving me an error message ” The plugin does not have a valid header ”
and I cannot find it on the plugins page then when i try to install it again it’s giving me an error message “installation failed: Destination folder already exists. /public_html/wp-content/plugins/akismet/”
when i try to update it “Update Failed: Could not remove the old plugin”
i had tried to delete the old plugins folder and replace it with a backup, the file permission is correct 755 and i tried also 777 but it’s not working also and i tried to deactivate all the plugins and also not working

The page I need help with: [log in to see the link]