when i install new plugin error occurs it says
"error output so disabling automatic redirect"
what should i do?
It's not possible to install plugins on MoodleCloud and error output is forcibly turned off.
Unless I'm mistaken, you're posting in the wrong forum - please ask in the Installing and upgrading help forum (unless a moderator comes in and moves this discussion over).
Now, to help answer your question, the message indicates that "Error output" has been turned on (that's a preference in your configuration under /admin/settings.php?section=debugging in your Moodle site - and which should never be done on a production site) and that automatic redirection are prevented.