Your mom can't be a dependent on your return if she has already filed a tax return.  You can remove her off your return and you will be able to e-file.  

If she did not file a return or erroneously filed a return, you can claim her, but you will have to file by mail.  You won't be able to e-file with her on your return.  And if you file by mail and keep her as a dependent, she will need to file an amendment to ''unclaim'' herself.  

Everyone can only be claimed on 1 tax return.  Here are the rules for claiming Qualifying Relatives on your tax return.  

Qualifying relative

• They don't have to be related to you (despite the name).
• They aren't claimed as a dependent by someone else.
• They are a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
• They aren’t filing a joint return with their spouse.
• They lived with you the entire year.
• They made less than $4,150 in 2018.
• You provided more than half of their financial support.