Can't add second Google account on my phone

[FezzantPlucka]

My mum has just given me her old phone that hasn't been used for several years. It's a Galaxy J3 2016. Unfortunately it has her GMail account on it so I want to delete her off and add mine instead. However the phone won't let me.

The phone has accepted my sim card and I can use the "phone" part no problem.

I have a Google and Gmail account set up and working.

If I go: Settings>Accounts>Google>Add account> I then tap Google but nothing happens. It's like I never tapped the button.

If I go: Apps>Google>Gmail it starts trying to access my mum's Gmail account and doesn't give the option to choose or create another one.

If I try to delete her with: Settings>Accounts>Google>Her Account>More>Remove Account that does nothing. I have heard android phones require at least one google account to be on the phone so is that the reason?

Additionally, if all else fails and I choose to factory reset the phone could that be a solution? One of the tutorials I read states I need to know the password to a Google account that exists on the phone. However the only one on there is my mum's, which would imply the phone would just go back to her again. Surely a factory reset should put everything back to zero and I can just use my own account from that point?

Thanks.