Let's go piece-by-piece:
I can access from my firestick, and strangely, I can access from my Android phone.
I'm not sure, but think firestick and the Android phone can access because they are by-passing pi-hole.
I have tried whitelisting peacocktv.com domain
This domain and related domains/subdomains are whitelisted using this 3 entries:
(\.|^)peacocktv\.com$ovp.peacocktv.compeacocktv.com
I temporarily disabled Blocking and tested with Chrome. I was able to login which shows that it is indeed pihole that is blocking
I've since reenabled blocking but noticed that I can still access my peacock tv accounts since I never closed that tab. My second browser, Edge, still cannot access peacock tv.
This probably means the peacocktv.com domain is correctly whitelisted, but a different domain is needed to login and this domain might be blocked by pi-hole.
To find out what is being blocked you can use the Pi-hole Query Log (pihole -t on the command line or http://pi.hole/admin/taillog.php on the web interface). More details here.