Signing Plugins , grafana/toolkit created empty manifest.txt file

I installed foursquare-clouderamanager-datasource as a external plugin, when i tried to sign it with grafana/toolkit stderr was like that;
npx @grafana/toolkit plugin:sign
Building manifest…
Signing manifest…
Saving signed manifest…
Signed successfully
after restart grafana server when i cheked cloudera in grafana it was 'invalid signature after checking MANIFEST.txt it was empty.

Any tips would be great.

Thank you.