Unable to set canonical for Magento 2 homepage

Add a canonical link to homepage with existing theme Ultimo:

  1. app/design/frontend/Infortis/ultimo/Magento_Cms/layout/cms_index_index.xml
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
    <head>
        <link rel="canonical" src="https://www.example.com" src_type="url" />
    </head>
</page>

OR

You can try following code with new module:

  1. app/code/[VendorName]/[ModuleName]/registration.php
\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::MODULE,
    '[VendorName]_[ModuleName]',
    __DIR__
);
  1. app/code/[VendorName]/[ModuleName]/etc/module.xml
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="[VendorName]_[ModuleName]" setup_version="1.0.0"/>
</config>
  1. app/code/[VendorName]/[ModuleName]/view/frontend/layout/cms_index_index.xml
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
    <head>
        <link rel="canonical" src="https://www.example.com" src_type="url" />
    </head>
</page>

<link> elements are not allowed in the homepage Layout Update XML.

You can add a canonical link to your homepage by creating or editing cms_index_index.xml in

app/design/frontend/{vendor}/{theme}/Magento_Cms/layout/

<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
    <head>
        <link rel="canonical" src="http://domain.com" src_type="url" />
    </head> 
</page>

Clear static files and cache and you're done.