There can be multiple reasons why your Signature was rejected.
To know why your Signature was rejected, kindly check your registered email. Here is an example of an email for your reference:
If you are unable to view this email in your inbox, please check your spam folder as well.
If your Signature is rejected, you can proceed to re-upload from the Web or re-draw on the app by clicking on the "Upload your Signature" button in the email.