My Apple TV is saying the WiFi password I've entered is incorrect.

early generation Apple TV.

Please find the exact model.

Identify your Apple TV model - Apple Support

Find your Apple TV model number - Apple Support

I get a message saying that the password I've entered is incorrect.

1. Try the steps as mentioned in: If your Apple TV can't connect to Wi-Fi - Apple Support
2. Try the ‘Other…’ option instead of selecting a listed discovered network.
3. For Apple TV (4th generation or earlier), verify that the router or access point isn’t set to use WPA3, as only device models from ~2018 or later support that. Instead use ‘WPA2/WPA3 Transitional’. Some users report that they had to set ‘WPA2’-only first.

2. Find the ‘Other…’ option below the list of discovered networks.

3. You would set the WPA version on the router or access point. Many routers have web access to its settings through its LAN IP address (e.g. http://192.168.1.1/, but the 3rd number can be anything else (0–255) as configured for your home network. Check on your devices that do connect.* Naming of menus and settings can have variations per brand and model. E.g. on my router, this setting is accessible by navigating to Wi-Fi﹥Security﹥Encryption﹥, but yours could be different.

* E.g. on iPhone, your router IP address would be listed in: Settings﹥Wi-Fi﹥[current connected network name] ⓘ﹥[ɪᴘᴠ4 ᴀᴅᴅʀᴇss] Router:﹥ [its IP address]