[@davesumpter Participant 10 hours, 42 minutes ago]

Try this in the functions.php file of your child theme. I have put 777 as filed id but you will have to put in the correct field id and the correct field name:-

function validate_required_profile_fields() {
$required_fields = [
‘field_777’ => ‘The so and so field is required.’,
];

foreach ( $required_fields as $field_key => $error_message ) {
if ( empty( $_POST[ $field_key ] ) ) {
bp_core_add_message( $error_message, ‘error’ );
}
}
}
add_action( ‘bp_signup_validate’, ‘validate_required_profile_fields’ );