The key "<your key>" under "env" in `next.config.js` is not allowed.

Why This Error Occurred

Next.js configures internal variables for replacement itself. NEXT_RUNTIME along with variables starting with __ or NODE_ are currently internal, for this reason they are not allowed as values for env in next.config.js

Possible Ways to Fix It

Rename the specified key so that it does not start with __ or NODE_, or pick another name for NEXT_RUNTIME.