I faced this issue:
https://help.heroku.com/MDM23G46/why-am-i-getting-an-error-when-i-upgrade-to-pg-8
The node version used is 14.15.5. Here some of the build logs with the corresponding info:
-----> Building on the Heroku-18 stack
-----> Node.js app detected
-----> Creating runtime environment
NPM_CONFIG_LOGLEVEL=error
USE_YARN_CACHE=true
NODE_ENV=production
NODE_MODULES_CACHE=true
NODE_VERBOSE=false
-----> Installing binaries
engines.node (package.json): unspecified
engines.npm (package.json): unspecified (use default)
engines.yarn (package.json): unspecified (use default)
Resolving node version 14.x...
Downloading and installing node 14.15.5...
Using default npm version: 6.14.11
Resolving yarn version 1.22.x...
Downloading and installing yarn (1.22.10)
Installed yarn 1.22.10