from typing import List
from typing import Dict


def get_required_tag_value(tag_set: List[Dict[str, str]], key: str) -> str:
    """Get a tag value from a TagSet. Raise ValueError if the key is not present.

    Args:
        tag_set: list of dicts, each of which contains keys 'Key' and 'Value'.
        key: tag key string

    Returns:
        tag value string

    Raises:
        ValueError if key is not present in tag_set
    """
    for tag in tag_set:
        tag_key, tag_value = tag["Key"], tag["Value"]
        if tag_key == key:
            return tag_value
    raise ValueError(f"Required tag key {key} not found in {tag_set}")