What can I use to track online training completion?

Generally one uses a Learning Management System or LMS.

Can be as simple as an online 'attestation' form, or a complex as measured assessments reporting to a learning management system.

Some tools have created some neat "reporting lite"- like Wondershare- where completions/quiz scores are emailed to a manager or posted to a database.

If you want to share specifics, please feel free to reach out, but I would offer- apply the KISS principle. Seen too many companies deploy expensive solutions where they only needed a fraction of the functionality which was competently offered by a lower-cost alternative.