Warehouse Chief Ranspo:
To qualify to become a Scavenger, your level must be at least 20 and you must pass the Test of the Scavenger. However, you haven't met either the level or the test requirement yet.
Go raise your level and take the Test of the Scavenger. Ask Collector Pippi of the Dwarven village about the test. She will tell you everything you need to know about it in detail. After you have passed the test and increased your level, come back to me. Then I will change your profession to a Scavenger.