Zakat is payable on strong debts, i.e. money that is owed to you that you are confident will be paid. This may include personal loans to friends and family. This does not include outstanding wages, dowry, inheritance or assets held in trust (other than assets held under a Bare Trust).
A debt that is owed to you is still classed as an asset depending on certain criteria being met. It is classed as a personal asset because you are likely to get it back.
See Personal liabilities, debts and loans for other debts that are owed by you.