Hello. I love in Illinois and run a vocational school. If a prospective student cannot afford my tuition, I would like to file a lien against their professional license. How do I go about doing this?

You can't. There is no such thing. A lien is an encumbrance placed against a physical asset to prevent the sale of that asset until the lien is satisfied. A professional license is NOT a physical asset, nor is it sold.

You can, however, withhold the license until payment has been made.

You can get a judgement against the person, You can not give them their graduation until it is paid. Or require them to get outside funding. Perhaps work with a local loan company.