Probably because you are trying to use the hex format. Use the nsec instead
Discussion
How do I convert it to nsec?
The normal way: https://nostrcheck.me/converter/
The hard way: https://github.com/rot13maxi/key-convertr
So you’re telling me that if someone doesn’t know how to do it the hard way, then they have to input their private key directly into a website?
When you make a nostr account now does it give you both hex and nsec?