when I am trying to open erp it says certification error
Have you set up letsencrypt certificate? Note that it also expires in 3 months, so you need to renew it after that.
from where I can renew it
To renew certificates manually you can use:
sudo bench renew-lets-encrypt
(remember to make the command in frappe-bench directory)
Read more on this:
here’s one post on automating renewal