my guess is that you claimed the child that she claimed. you can no longer e-file due to duplicate claimants for the same child. eventually, The IRS will send each of you a request to amend. she needs to change her return to a filing status of single and not claim the child. It seems that based on the little info provided, you can claim both your GF and the child as dependents. Your filing status would be head of household.
you can claim the GF as a dependent if the qualifying relative tests are met. even though the "relative" is used, in fact, you can anyone that meets all these tests:
1) their gross income for 2019 less than $4,200
2) you provided over ½ their support
3) they aren't a qualifying child of another taxpayer
4) they lived in your household for the entire year or meet a relationship test
the stimulus payment you would get is $1200 for you and maybe $500 for the child. (not sure what effect your GF claiming the child will have on you getting the stimulus for it)
your GF should have gotten the $1200 for herself and $500 for the child she claimed.
