I tried to use the /function command in two versions: 1.12.2 and 18w05a. First, I created a function named test,like this: data/function/test/test.mcfunction.

I entered in the world on 1.12.2 version, used the /function command and everything was normal. After this, I tried to do the same on 18w05a, but the function didn't worked (Unknown function test:test).