tracking code hook - Unable to execute PHP as DISALLOW_FILE_EDIT is defined.

I'm trying to add a hook with site tracking code from my email service provider to the footer of my site.

When I create a new hook element I'm seeing "Unable to execute PHP as DISALLOW_FILE_EDIT is defined".

I've removed all plugins except Generateblocks, Generateblocks Pro and GP Premium.

I'm still seeing the error.

I've cleared my browser cache and disabled my CDN.

I checked the wp-config.php file and I can see the following code:

define( 'DISALLOW_FILE_EDIT', FALSE );

which I think means that disallow file edits should be turned off? I'm confused. And stuck.

What else could I try?