"Safari cannot connect to the server." This is what keeps popping up on my sister's 2014 Macbook. [View all]

We use verizon wifi jetpaks. The guy at the computer store could connect it to the wifi in his store. I can connect to wifi in our house with my PC, phone, kindle. We are kind of technically challenged, so if anyone could explain in simple terms what might be the problem. Or do we need to get a new Macbook? She has cognitive issues and needs something that will not be like learning a whole new system. Thank you.