As of Jira v10.3, users are no longer able to #parse to use additional velocity template .vm files. This forces Admins to now build all their customizations into the current .vm files supplied by Jira.

This functionality was allowed in Jira up until v10.3.

Example of Jira mentioning resource not found when using a new .vm file:

Workaround is to build all details from those files into the current email / velocity templates:

Workaround :
Modify the $JIRA_INSTALL/atlassian-jira/WEB-INF/classes/velocity-email.properties and add the below property -

resource.loader.file.allowlist = Your custom files separated by comma (,)