Can I use prepaid phone minutes without a monthly plan?

We have an AT&T flip phone we bought for our daughter for emergencies. She transitioned to a full iPhone and now we would like to active the phone for our son. This is to be used for emergencies only, like calling me if he's in a ditch. I already bought and installed the AT&T prepaid 3-in-1 SIM card and she has used her phone number so we need a new one.

Once I registered it, the prompts said I needed to buy a monthly subscription. I even went to the AT&T store before I did this and they sent me to Walmart instructing me to buy this plus an AT&T prepaid minutes card. Now I feel like I've wasted time and money. Can I just use the prepaid minutes as I thought I could? Help please!