What to do if the person cannot sign the DPOA?

Guardianship through the court. It's unfortunate, but if mom isn't competent, that's the only option. If she's competent but can't sign, an elder law attorney can help.

Is she physically unable to sign or is she not competent?

Does she have to sign in front of a Notary?

kinger56, she can not sign in front of a Notary. You go to Muskegon County Surrogate's court and enter a Petition for Guardianship. Contact an attorney who can help you through the process. No other way.

If she is only physically unable to hold the pen, she can give her assent in other ways and it can be witnessed, and those people can sign an affidavit that they witnessed her assent, and those people cannot be beneficiaries to the documents being signed. And this all has to be notarized. And notaries do make house calls.