How can a I reset my user password (osx external ssd)

Hi,

Im running BigSur (11.7.4) on an external ssd connected to an iMac.

This osx have autologin, so I dont need to login, I have access to the desktop.

But I dont remember the user pass.

• I try to reset the password on the osx but is asking me for the old one

• Doesnt have any icloud attached, I tried to attach one but is asking me for the osx password.

• I tried to run on terminal resetpassword but said the command doesnt exist

• I boot in recovery mode (command+R) but goes to the original computer harddrive no the external one

Any ideas? Thanks