This exercise wants you to write two functions. To ensure that your solution is correct the tests call both functions and check if the result is correct.
Usually C++ code where a function gets called but is not defined does not compile.
The author of this exercise did try to allow solving it incrementally and being able to run the tests without having to write all functions before.
That did work at first but apparently not anymore, now you get this error message that is not really helpful. (We will have to modify the tests.)
But in the meantime you can view that error message as some sort of cryptic compiler error. To get past it, define the two required functions. They do not have to be correct initially, they only need to be syntactically correct, so that the code compiles and the tests can run.
Yes, you just need to add the second function assemble_code(){} into the namespace estate_executor, fixes everything as far as gross compiler errors go.