After updating to WHMCS 8.6.1, you may be unable to select PHP Mail for Mail Provider in the Mail tab at Configuration > System Settings > General Settings.

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/185-core-18329-switch-from-an-external-mail-provider-to-php-mail/

Run the following command to reset the Mail Provider setting in the WHMCS database:

UPDATE tblconfiguration SET value='' WHERE setting='MailConfig';

Then, reload the Mail tab at Configuration > System Settings > General Settings and reconfigure your email settings.