In addition to all that's gone before:
Serial.readString is likely not going to do what you want here. It does not return when the Enter key is pressed. It returns when it times out (which, by default, is set to 1000 mSec). If you don't type fast enough, you'll only get part of your input. If you do type fast enough, you'll likely get the character representing the Enter key if you press that out of habit at the end of typing your password.
Slightly better would be to use Serial.setTimeout to give yourself much more time between keystrokes, and to use Serial.readStringUntil to read characters up to the Enter key being pressed. (I leave it to you to discover what that character is.)
Much better would be to come up with your own input routine. Something that assembled keystroke characters into a String w/o timing out, and finishing only when it detected the character from the Enter key.
Reading @Robin2's Serial Input Basics tutorial is highly recommended.