How do I "sign in" to the Brave Broser

You cannot sign-in to any Brave service in the way you would on Chrome.
The issue you’re seeing with keep is related to a conflict between Brave’s protections and installed extensions. This issue is actively being worked on.

Additionally, this question has been asked several times before:

We’re always happy to see new members joining the Community and getting involved in the discussion. However, I have to you to please follow the Posting Guidelines/Template when you request support here. The guidelines are there to ensure that we have all the information we need up front and we can provide support efficiently.