Hi and welcome to the community. 
Try this:
ISSUE: 400 Error w/ Various Google Services
Preferred Workaround: Disable Enable First Party Ephemeral Storage in brave://flags
A fix is in the works. Once implemented, change flag back to default.
Sign in issues and 400 type error - #3 by Mattches
Brave shields are blocking Gmail - #9 by Mattches
Another Workaround: Google Services/Logins No longer work with Brave - #2 by Mattches
More involved workaround below. Try this if the above workaround doesn’t work.
Cannot Login/Error 400 posted by @289wk