bought 0.1 BTC in 2013, lost access to my wallet, and just found it on an old laptop

Three days ago, I was clearing out my garage and found an old Toshiba laptop from college. I almost tossed it, but for some reason decided to boot it up.

After 20 minutes of watching that Windows Vista loading circle, I noticed a folder labeled 'Important Stuff' with a text document inside.

The document contained what I now know is a private key for a Bitcoin wallet, along with a note I'd written to myself: 'Don't lose this $100 worth of magic internet money.'

At first, I didn't think much of it. But something made me download Electrum and import that key. When the blockchain finished syncing and I saw the balance, my hands started shaking.

It's still there. All 0.1 BTC. At today's prices, that's over $6k.

But here's the thing, I can't move it. Why?

I'm starting to think there's something wrong with the key format or maybe the wallet structure has changed too much since 2013.

Has anyone ever experienced this? The irony is killing me, I find my lost treasure only to discover I can't open the chest.

DMs are open if you've got a brain that works.