Email template 'custom_mail_template' is not defined
You need create file email_templates.xml
in etc
folder with content:
<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Email:etc/email_templates.xsd">
<template id="custom_mail_template" label=" Your Custom Label" file="yourfile.html" type="html" module="Vendor_YourModule" area="frontend"/>
</config>
This file using declare template have id custom_mail_template
in Configuration with path of template file you want use.
After, you create template email file yourfile.html
in path Vendor/YourModule/view/frontend/email/yourfile.html
I hope it help for you.