Your PI should check the Spam/Junk folder in the mailbox; if the link weren't received, the PI can use the following link to see the pending requests and approve them:-
For the Farm cluster PIs, check the below link:
https://hippo.ucdavis.edu/Farm/approve
For the Peloton cluster PIs, check the below link:
https://hippo.ucdavis.edu/Peloton/approve
For the Franklin cluster PIs, check the below link:
https://hippo.ucdavis.edu/Franklin/approve