After updating to WHMCS 8.6.1, you may see a TypeError error when sending Email Marketer rules during the automation tasks:

TypeError: array_search(): Argument #2 ($haystack) must be of type 
array, null given in 
/path/to/whmcs/vendor/whmcs/whmcs-foundation/lib/Cron/Task/EmailMarketer.php:0

An invalid value in the Email Marketer rule (missing billing cycle) causes an error in PHP 8.1 environments only.

To work around this issue:

1. Go to Utilities > Email Marketer .
2. Edit the first Email Marketer rule.
3. Without making any changes, click Save Changes.
4. Repeat this process for each Email Marketer rule.

After you do this for all of your Email Marketer rules, the error will resolve.