If you need to activate debug but you cannot access moodle admin page, this article is for you:

Step 1: note original settings

SELECT name, value
FROM mdl_config
where name ='debug' or name ='debugdisplay'
;

for instance :

debug   0
debugdisplay    0

Step 2 : setup debug config in db

UPDATE mdl_config SET VALUE = 32767 WHERE name = 'debug';
UPDATE mdl_config SET VALUE = 1 WHERE name = 'debugdisplay';

Step 3: clear cache from CLI

php  moodle/admin/cli/purge_caches.php

Step 4: restore original settings

UPDATE mdl_config SET VALUE = 0 WHERE name = 'debug';
UPDATE mdl_config SET VALUE = 0 WHERE name = 'debugdisplay';