I got an error on my taxes said my mom claimed me however I am 20 years old help

There are two types of dependents: qualifying child and qualifying relative. If you meet the requirements to be claimed as either type of dependent, you can't claim your self and someone else can claim you.

Qualifying Child

1. The child must be your son, daughter, stepchild, foster chld, brother, sister, half brother, half sister, stepbrother, stepsister, or a descendant of any of them.
2. The child must be (a) under age 19 at the end of the year, (b) under age 24 at the end of the year and a full-time student, or (c) any age and permanently and totally disabled.
3. The child must have lived with you for more than half of the year. Temporary absences while away at college are considered living with you.
4. The child must not have provided more than half of his or her own support for the year.
5. If the child meets the rules to be a qualifying child of more than one person, you must be the person entitled to claim the child as a qualifying child.
6. The child must be a U.S. citizen or U.S., Canada or Mexico resident for some portion of the year.
7. The child must be younger than you (or your spouse, if filing jointly) unless disabled.

Qualifying Relative

1. The person cannot be your qualifying child or the qualifying child of any other taxpayer. A child is not the qualifying child of any other taxpayer if the child's parent (or any other person for whom the child is defined as a qualifying child) is not required to file an income tax return or files an income tax return only to get a refund on income tax withheld.
2. The person either (a) must be related to you or (b) must live with you all year as a member of your household.
3. The person's gross taxable income for the year must be less than $4,200 in 2019.
4. You must provide more than half of the person's total support for the year.
5. The person must be a U.S. citizen, U.S. resident alien, U.S. national, or a resident of Canada or Mexico.

If you meet the requirements for either type of dependent, you must check the box on your tax return that you can be claimed by someone else.

If you don't meet the requirements and someone claimed you who should not have claimed you, you will have to print and mail your return to get it accepted. 

Later in the year the IRS will send notifications to both you and the other person who claimed you requesting documents to support your claim. If you win, the other person will have to pay back any tax benefits from incorrectly claiming you. If you lose, you must pay back any benefits your received and shouldn't have.