There can be any number of reasons, but the most likely is that you claimed too many allowances on your Form W-4.
Claiming too many allowances results in too little withholding from your paycheck, and eventually owing money once you file your tax return.
Another possibility that's very common this year is if you received unemployment benefits, but had no taxes withheld from it.
We're unable to see your return in this forum, so we can only speculate.
Please see the resources below for more information.
How do I update my 2020 withholdings and print a new W-4?
What to know about the new 2020 IRS Form W-4 Employee’s Withholding Certificate
Video: Why Do I Owe on My Tax Return?
