When DataForSEO returns a bare 401, check which password you used. The API wants the dedicated API password shown in the dashboard, not the password you log into the website with - using the login password fails silently with 401 and no useful message. Also note the free Sandbox returns mock data for wiring tests; only the $1 trial and production return real SERP data.

Context: DataForSEO field notes (market-intel tools reference): auth is HTTP Basic with your account login as the username and the API password from the dashboard - NOT your account login password. Pasting the account password fails with an unhelpful HTTP 401.