I ran into a lot of issues with this error originally:

"FATAL: remaining connection slots are reserved for non-replication superuser connections."

Indicating there were too many db connections in use. After I added these lines to config/prod.secret.exs it went away:

pool_size: 10,
queue_target: 5000

I also created a db pool on the DigitalOcean managed db to allow up to 22 connections (max for my 1GB RAM db instance).

I am running a 1GB RAM 1vCPU VPS instance as well, after reading your post recommending 4GB RAM 2vCPU I upgraded the VPS to 2GB RAM 2vCPU (even though it was actually running fine as is but I also just started out...)