After updating to WHMCS 8.6.1, you may see a TypeError error when you access the client profile Emails tab for a client with a failed Email Campaign message:

TypeError: count(): Argument #1 ($value) must be of type Countable|array, null given in /path/to/whmcs/admin/clientsemails.php:0

Update to the WHMCS 8.7 release at Utilities > Update WHMCS.

We have created a hotfix for WHMCS 8.6.1 to resolve the error:

https://whmcs.community/files/file/182-core-18323-typeerror-in-clients-email-tab/

Update your hosting environment to use PHP 7.4 or run the following command to delete the failed entry from the tblemaillog table:

DELETE FROM `tblemails` WHERE `failed` = 1 and userid = x;

In this example, x is the client's ID.