Then your user password isn't set to what you think it is. So you'll have to log in as root and "passwd <user>" replacing <user> with your username.
If you know the root password, you can probably become root by typing "su"
If you don't know the root password, you'll have to boot single user mode (does Linux do that?) Or, boot the same operating system from another drive (SD card versus eMMC or maybe USB) and manually edit your /etc/shadow file to delete the password. I'm assuming that even works in Linux; I haven't manually edited a shadow file since Solaris.
Also important: if you're using Manjaro, make sure to fully update the operating system. In some versions, the keyboard settings will be different when you login compared to after log in. If you can login with a password but can't authenticate after login, that could be the issue. You might be typing gibberish letters and don't even know it because the password is hidden when you're typing it at login.
If you know the root password, you can probably become root by typing "su"
If you don't know the root password, you'll have to boot single user mode (does Linux do that?) Or, boot the same operating system from another drive (SD card versus eMMC or maybe USB) and manually edit your /etc/shadow file to delete the password. I'm assuming that even works in Linux; I haven't manually edited a shadow file since Solaris.
Also important: if you're using Manjaro, make sure to fully update the operating system. In some versions, the keyboard settings will be different when you login compared to after log in. If you can login with a password but can't authenticate after login, that could be the issue. You might be typing gibberish letters and don't even know it because the password is hidden when you're typing it at login.