Your connection is not private

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