Are you literally using that as the key?
The docs Channel Configuration | Meshtastic
Must be either 0 bytes (no crypto), 16 bytes (AES128), or 32 bytes (AES256).
BUt I think you need to enter it as base64 encoded. [Bug]: Documentation issue regarding channel keys · Issue #3855 · meshtastic/firmware · GitHub
The Android app has a button to generate a random, but valid key. Use the QR code to transfer between devices.
Base64 decoding xxxyyy=== is only 4 bytes. But it also doesnt look actully valid decoding.