Gives error, also shows update available but fails and gives another error

I installed it and seems to be working..
It did give me an error though:

"The plugin generated 2 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin"

It also notified that there was an update, but trying to update
it fails and gives another error:

 Update Failed: {"success":true,"data":{"update":"plugin","slug":"woocommerce-popup-extension","oldVersion":"","newVersion":"","plugin":"woocommerce-popup-extension\/extension-plugin.php","pluginName":"WooCommerce Pop-Up"}}