The initial values aren’t actually drawn from the prior - they are drawn (I believe, depending on the backend!) from N(0, 1). So this error is to be expected and shouldn’t be an issue, as rejected initial values are just tried again.
If you would like to constrain initial values, you can pass the “init” argument to the brm function. It expects a list of named lists, with a value for each parameter. But - you don’t need to do this unless this happens repeatedly and your chains fail.
An example of passing inits is in this post: Does the init argument work with cmdstanr? - #2 by jonah