Azure Devops YML Conditional Stage Template

Visual Studio 2019 16.8.5

I want to use template for stages. Some stages, I'd like to be based on conditions. However, when I try this in the YML file, it generates errors. Here are two attempts in the azure-pipelines.yml file:

stages:
- template: 'build.yml'

- template: 'deploy.yml'

- template: 'test.yml'

- ${{ if eq('true', 'true') }}:
  - template: 'optional.yml'

Results in:

Severity    Code    Description Project File    Line    Suppression State
Error       Property ${{ if eq('true', 'true') }} is not allowed.       azure-pipelines.yml 8   

And this:

stages:
- template: 'build.yml'

- template: 'deploy.yml'

- template: 'test.yml'

- template: 'optional.yml'
  condition: eq('true', 'true')

results in:

Severity    Code    Description Project File    Line    Suppression State
Error       Property template is not allowed.       azure-pipelines.yml 8   

What is wrong with the above syntax and what is needed to achieve the requirement without errors?