Intermediate YAML template, resources repository's ref and parameters "Internal error reading the template. Expected a scalar,a sequence,or a mapping" - azure-devops

Question

Azure Pipelines docs state that we can use template expressions in resources.repositories repository.ref argument.
It already works in some parent template, but using $(Build.SourceBranchName) instead of template expressions. In some child template, the template expression produces the error "Internal error reading the template. Expected a scalar, a sequence, or a mapping". BTW, if I use template expressions in parent template, I get there the same error.
Alternatively, if I use $(Build.SourceBranchName), child template throws error "Could not get the latest source version for repository xxxx-dotnet-solution hosted on Azure Repos using ref $(Build.SourceBranchName). (in this case, Azure Pipelines won't parse the variable).
I can't figure out how to fix this problem.
Here's the affected child template:
parameters:
- name: buildConfiguration
type: string
values: [Debug, Release]
- name: api
type: string
- name: solutionLocation
type: string
resources:
repositories:
- repository: xxxx-apis-parent-templates
name: xxxx-dotnet-solution
type: git
ref: ${{ variables['Build.SourceBranchName'] }}
extends:
template: .az-devops-cicd/templates/aks.yaml#xxxx-apis-parent-templates
parameters:
solutionLocation: ${{ parameters.solutionLocation }}
projectRepository: xxxxxx-apis
projectBaseDir: ${{ parameters.solutionLocation }}/${{ parameters.api }}/
projectName: XXXX.${{ parameters.solutionLocation }}.Apis.${{ parameters.api }}
buildConfiguration: ${{ parameters.buildConfiguration }}