Hello. I was given a used TI-84 plus Silver Edition to use to prepare for nursing school, however, when I tried to use it, two different messages came up and I cannot clear either one to do anything else.
“ERROR! Press any key to turn unit off then turn unit back on. “
But when I do that, this message comes up,
“Waiting... please install operating system now”
I have no idea what to do. I have removed all of the batteries and replaced them with new ones, including the lithium battery. Nothing works. Is there somewhere I can take it to be repaired? I’m not particularly tech savvy when it comes to these things. Thank you so much!

A good way is to use TI Connect. It's for computers. Go on education.ti.com to get it and get the Operating System for your specific calculator. Just send it using TI Connect. You are missing an Operating System on the calculator.

Download 2.55MP for your calculator from education.ti.com, then TI-Connect it to your system. Reserve about an hour for it to validate.

If you'd like a video tutorial, here's one on YouTube!