I suppose this would be interpreted as "He could not find the soap", right?
Yes it could, and it's an accepted answer too.
The 'e" in 'tvålen' should be short.
Note to all: The TTS used to be bad with this, but the new TTS says it properly.
Is there a way to clarify between He didn't find it , and He couldn't find it in Swedish?
Indeed! Like this.