 ```python
def fill_template(template: str, values: dict) -> str:
    if not isinstance(template, str):
        raise ValueError("Template must be a string")
    
    result = []
    i = 0
    while i < len(template):
        if template[i] == '{':
            j = i + 1
            while j < len(template) and template[j] != '}':
                j += 1
            if j == len(template):
                raise ValueError("Unterminated placeholder")
            if i == j or not template[i+1:j].isalnum():
                raise ValueError("Invalid placeholder name")
            placeholder = template[i+1:j]
            if placeholder not in values:
                raise ValueError(f"Unknown placeholder '{placeholder}'")
            result.append(values[placeholder])
            i = j + 1
        else:
            result.append(template[i])
            i += 1
    
    return ''.join(result)
```