Hi @faznar,
I just launched a fresh instance of the Mautic solution and enabled the API without problems
I found that you modified the permissions of the application and that's why it can't write it.
apps/mautic/htdocs/app/config:
total 92
drwxrwxr-x 2 bitnami daemon 4096 Aug 31 13:07 .
drwxrwxr-x 7 bitnami daemon 4096 Aug 31 13:04 ..
-rw-rw-r-- 1 bitnami daemon 2310 Aug 31 11:55 config_dev.php
-rw-rw-r-- 1 bitnami daemon 10972 Aug 31 11:55 config.php
-rw-rw-r-- 1 bitnami daemon 1896 Aug 31 11:55 config_prod.php
-rw-rw-r-- 1 bitnami daemon 4399 Aug 31 11:55 config_test.php
-rw-r--r-- 1 bitnami daemon 17274 Sep 22 18:02 local.php
The original configuration is
bitnami@bitnami-mautic-c85b:/opt/bitnami/apps/mautic/htdocs$ ls -la app/config/
total 92
drwxrwxr-x 2 root daemon 4096 Aug 31 13:05 .
drwxrwxr-x 7 root daemon 4096 Aug 31 13:02 ..
-rw-rw-r-- 1 root daemon 2310 Aug 31 11:55 config_dev.php
-rw-rw-r-- 1 root daemon 10972 Aug 31 11:55 config.php
-rw-rw-r-- 1 root daemon 1896 Aug 31 11:55 config_prod.php
-rw-rw-r-- 1 root daemon 4399 Aug 31 11:55 config_test.php
-rw-r--r-- 1 daemon daemon 17180 Sep 25 12:04 local.php
Let's try to solve this issue by changing the permissions of the local.php file
sudo chown bitnami:daemon /opt/bitnami/apps/mautic/htdocs/app/config/local.php
sudo chmod 664 /opt/bitnami/apps/mautic/htdocs/app/config/local.php