Answer|Solution:
This problem is caused by invalid whitelist rules in the “Plugins Script|File Whitelist Text Area” textarea box. This error check is static to prevent problems that would occur if you activated the Plugin Firewall with invalid whitelist rules. This error check will not allow the Plugin Firewall to be activated/Turned On until the whitelist rules are corrected.
To fix/correct this problem go to the Plugin Firewall feature. You will see an inpage error message that explains exactly what the problem is. In your case the error message was – “Error: Your Whitelist rules either have additional/extra whitespaces between whitelist rules, line breaks/new lines between or after your whitelist rules or no space after the comma between your plugin scripts/paths/rules.”. I added the missing whitespace between a whitelist rule and the php error is no longer being generated.