Unable to unmarshal custom type hcl data

[RATANAJANGIR]

Struct format

type (
	Specification struct {
		Environment
		Templates
	}

	Environment struct {
		Build   image.Image
		Runtime image.Image
	}

	Templates struct {
		ArtifactPathTemplate          string                      `mapstructure:"artifact-path"`
		StartCommandTemplate          *executable.TemplateCommand `mapstructure:"start-command"`
		ComponentInstallationTemplate *executable.TemplateCommand `mapstructure:"component-install"`
		ExecutableBinary              string                      `mapstructure:"executable-binary"`
	}
)

getting error
error decoding 'Specification': unsupported entry[type= []map[string]interface {}, value= ]
Anyone can help how to decode these value