google play store authentication error
I am trying to purchase instagram app from google play. when I select to purchase, it tells me I have an authentication error and need to sign in to my google account, but does not give me login screen to do so. When I go to google screen to login, it shows my google account already logged in. I've read through their help screens, and see we are to sign in to authorize each purchase to avoid unwanted purchases. What am I missing, not doing or doing wrong? This is on my son's phone, he does have 2 accounts on it, but it does show his google account as the selected acct. I'm stuck! thanks.