This is an update from IRCC for applicants with one name. Posting over here so that it helps others in the same situation.
We’re currently working on fixing this issue, but it will take some weeks to implement. In the meantime, you can apply for an IRCC Portal account by entering your surname in both the “Given name” and “Surname” fields. However, once you have signed into the IRCC Portal, when you enter your name on your application, be sure to follow the instructions on the form. Once we fix the issue with the mandatory “Given name” field, you will be able to change your name on your IRCC Portal account by going into your profile from the main IRCC Portal home page.