See: https://calderaforms.com/doc/caldera_forms_validate_field_field_type/

caldera_forms_validate_field_dropdown.php

<?php
add_filter( 'caldera_forms_validate_field_dropdown', function( $value, $field, $form ){
	//return if not a specific form -- UPDATE form ID please!
	if( 'cf1234567' != $form[ 'ID' ] ){
		return $value;
	}
	
	if( 'Han Solo' == $value  ) {
		return new WP_Error( $field[ 'ID' ], 'Sorry we are sold out of Han Solo dolls right now.' );
	}

	return $value;
	
}, 25, 3 );

caldera_forms_validate_field_text.php

<?php
add_filter( 'caldera_forms_validate_field_text', function( $value ){
  if( 8 < strlen( $value ) ){
      return new WP_Error( '', 'Input must be longer!' );

  }
  return $value;
});