With errors like this it really is helpful to copy and paste the full error as well as the full stacktrace.

You’ll generally get this error when the application config is used by Application.compile_env, which generally means the value can indeed only be changed when compiling the application and not at runtime.

But the second argument to Application.compile_env can be a key or a path (unlike Application.get_env, which only takes a key). Doing Application.compile_env(:myapp, :key) means all the config nested within :key is considered compile time required.

If only a subset of that is actually required at compile time you can do Application.compile_env(:myapp, [:key, :compile]). That would make all the configuration nested at that path compile time required but e.g. config at [:key, :runtime] would be fine. One might be doing something like compile_time_value = Application.compile_env(:myapp, :key)[:compile], which would be needlessly tainting config values as compile time required. Those should be refactored to Application.compile_env(:myapp, [:key, :compile]).

A word of advice is to use credo, it will always warn you about defining compile-time config incorrectly in your code, and in general is a must-have tool for your elixir codebase, especially if you are new to the ecosystem.