@mingjih wrote:
Thanks for your answer, very clearly. My granddaughter live with my son more than half year. My son don't need to file return on last year for he income less. I pay more than 10% for my granddaughter. But I was rejected by e-File. If like this do I need to attach form 2120 to prove my son and me are also support my granddaughter?
In this case you can only claim your granddaughter if your son is not a "taxpayer" for 2019. He is not a taxpayer if he is not required to file a return and does not file, or only files to claim a refund of withholding and claims no other dependents, or credits.
Then, to claim your grandchild who does not live with you, you must have the agreement of every family member that provides at least 10% of the child's support. If that is only you and your grandson, that's easy. But if there are other relatives (especially a biological mother who is separated or divorced from the father) then it is much more complicated to get the agreement of everyone who pay more than 10% support.
If you get the agreement of everyone who pays more than 10% support, then you claim the child and you would be required to include form 2120 that lists the name, address, and social security number of every other person who could claim the child but is agreeing you let you claim the child. (that means in your case, everyone who pays more than 10% support.)
If you claimed the child as a dependent and were rejected, that means someone else claimed the child already. Either on a tax return, or on a "non-filer" form to get the economic stimulus. That non-filer form was really a special version of a tax return, and if your son sent in that form to claim his child as a dependent to get the extra stimulus payment, then you can't claim your child this year.
If someone else filed a dependent claim for the child, you would have to print and mail your return and the IRS will eventually investigate the duplicate claim. If it is someone with a better provable claim than you, you will lose the dependent credit.
Note that this type of dependent would get you only a $500 credit, not the $2000 credit.
