You must meet 2 requirements to open a Solo 401k:
1) Entrepreneurship - This can be working as a freelancer, independent contractor, or business owner. This can be in the form of a sole proprietor or a formal corporation or LLC. Your entrepreneurship can be brand new, part time, or full time.
2) No full time employees - If you pay yourself a W2 paycheck that's fine, but no outside W2 employees who work 1000+ hours a year.