what is the impact factor of "npj Precision Oncology"
For all such questions, go to Journal Citation Reports (JCR) and search for the impact factor. If you don't find the journal then it doesn't have one.

In the case of this journal though, you don't need to search JCR. It says on the website that the journal has been indexed by ESCI (Emerging Science Citation Index). This is a lower-ranking index, and if the journal has only just been indexed by ESCI, it certainly hasn't been indexed by SCI and SSCI (which are the indices that yields an impact factor). In other words, you can tell without searching that this journal does not have an impact factor.