In my old mirc version 6.03 I can use nicknames as an example nicknames with the letter Ñ
example ; nickname; niño or español

* Your nick is now niño << in status
* Drem is now known as niño << in channel
* Your nick is now español << in status
* niño is now known as español << in channel

I downloaded the mirc v7.61 and it won't let me use nicknames so niño or español in the status I get this error:
[12:43] niño Erroneous Nickname
some way or some way to fix it please ??? either configurations, it is just mirc since the ircd supports clear example is above

Sounds like you have a server which does not support UTF8. Try unchecking the choice: options/irc/messages/"utf8 encode message"
if it doesn't work, it would be hard to know without being able to test it at such a defective server too.