When you set up letsencrypt security certificate it asks if you want to redirect http traffic to https. Unfortunately, this causes an infinite redirect loop when you go to the admin page. You can start over by running the script again but this time don't redirect http traffic to https.
Im my case, after enabling letsencrypt, I had a FORCE_HTTPS=enable in my hosting settings and during magento install i had unchecked use HTTPS for admin, thats why i hade a redirect loop. Thanks for the information