Firstly, it is important to make sure your account has been activated. Please check if you have confirmed your registration using the link in the confirmation email.
Furthermore, your account needs to have been authorised for bidding. Please check if you have entered the verification code sent to you by phone or text. You also have to be signed in to place a bid. You can check the top right-hand corner of your screen to see if you are signed in; the page should show your username there.
Finally, your bid will not be accepted if it is invalid. 'Invalid bid' messages can occur when the amount entered is not high enough, when the amount entered includes a symbol other than a digit (such as a letter, comma or hyphen), or when the auction has already closed.