The eligibility requirements for the auto-enrollment credit (Part II of Form 8881) are different than the eligibility requirements for the startup credit (Part I of Form 8881). This has been expressed before on this forum in these posts months ago:
https://ttlc.intuit.com/community/business-taxes/discussion/re-how-to-claim-small-employer-auto-enro...
https://ttlc.intuit.com/community/tax-credits-deductions/discussion/re-how-to-complete-form-8881-for...
A sole proprietor with no W-2 employees, where the individual is the only person in the business, can have a Solo 401(k) plan with the auto-enrollment credit and claim that $500 credit each year for 3 years without qualifying for the startup credit.
This is further explained in this link which someone included in one of the above posts:
https://www.mysolo401k.net/the-self-directed-solo-401k-auto-enrollment-tax-credit-aka-auto-contribut...
This page explicitly explains how a Solo 401(k) with no W-2 employees can qualify for the auto-enrollment credit but cannot qualify for the startup credit:
https://www.mysolo401k.net/auto-enrollment-tax-credit-vs-the-start-up-credit-and-impact-on-solo-401k...
