Hello @heynoway!
At the moment Retool doesn't support conditionally hiding the header frame though it has been requested before and I can let you know here if it gets added. In the meantime, would it work to conditionally hide the navigation bar based on a value in {{ current_user }} (described here)?
Alternatively, it's possible to conditionally hide the header using Custom CSS as a temporary solution with something like:
header._wbjcJ._noBCS{
display: {{current_user.metadata.showNavigation ? "none" : "block"}}
}