Are you using the Email Template or Email Connector module from the Mendix Marketplace? Because since september 2022 the Email Template module is deprecated and replaced by the Email Connector module.
Having said that, on the SNIP_EmailTemplate_NewEdit page from the Email Connector module, it is possible to create new tokens. Please make sure you have the MxModelreflection module imported into your project and make sure the Model reflection has run for the modules that contain the entities you want to use for your tokens. If thats done you can add tokens to your email template. See this screenshot for refererence: