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