There was a modification in Spectra version, https://github.com/yixuan/spectra/blob/4215d760137ba0806df678fb8f8e6dea90e765c2/MIGRATION.md, and we cannot download flashpca with Spectra 1.0.0.
I was able to download flashpca by downgrading Spectra version into 0.9.0, but I think it should be noted somewhere in the documentation that flashpca only supports Spectra 0.9.0, or flashpca codes should be modified such that it supports Spectra 1.0.0.