Specifically, flash.NewQSPI is not defined for the feather-m4-can target.
However it is defined for the feather-m4 target.
Note that one contains the build tag atsamd51 while the other atsame51. The code block that defines NewQSPI is in flash/transport_qspi_samd.go and is guarded by the atsamd tag. I cannot tell yet whether that is truly atsmd specific, or if it can also be used on atsame -- or does atsame need its own flavor?