Click on "Initialize..."

or if the useer your login with can adminastrat the computer then try adding "sudo " and entering your password when prompted.

So:

iMac:~ User$ sudo diskutil eraseVolume HFS+ PenDrive disk2 password

Click on "Initialize..."

or if the useer your login with can adminastrat the computer then try adding "sudo " and entering your password when prompted.

So:

sudo diskutil eraseVolume HFS+ PenDrive disk2