fold method not working in angular v9.2.0

[pradeepkv-svec]

Hey, guys. I am using angular version v9.2.0 and "typescript": "~3.8.3" and "fp-ts": "^2.8.3" versions.I downloaded the code from this repository and placed the files in my local application.
I am facing the below errors in the tree.service.ts file.

and

The above _traverse and getNodeDepth are important methods ,without them-the tree will not come in the application.Can someone please help me on resolving these errors