If you are working on signing your Flutter App by following this guide Build and release an Android app and run in to this error. I hope this answer helps you.
In my case I changed the path to store my key.jks. This happened to me because I there was an existing file in that path.
keytool -genkey -v -keystore ~/key.jks -keyalg RSA -keysize 2048 -validity 10000 -alias key
This command stores the key.jks file in your home directory. To store it elsewhere, change the argument you pass to the -keystore parameter.
In my case,
keytool -genkey -v -keystore /Users/Y/Desktop/X/key.jks -keyalg RSA -keysize 2048 -validity 10000 -alias keyYour
i.e Y - Username and X - folder name
Then you will be prompted to Enter keystore password: and Re-enter new password:. Here you can use whatever password you want.
However, keep the keystore file private; don’t check it into public source control!