The following error occurs when a certificate in the chain of trust uses a critical extension that pkg does not understand.
pkg install: The certificate whose subject is /C=US/ST=California/L=Menlo Park/O=pkg5/CN=cs2_ch1_ta3/emailAddress=cs2_ch1_ta3 could not be verified because it uses a critical extension that pkg5 cannot handle yet. Extension name:issuerAltName Extension value:<EMPTY>
Until pkg learns how to process that critical extension, the only solution is to regenerate the certificate without the problematic critical extension.