langchain RetrievalQA.from_chain_type not working. It's showing ValidationError: 1 validation error for LLMChain

Question

I am fresher in the prompt engineering. Recently, I am facing a problem in the langchain PromptTemplate. I'm using jupyter notebook. And I want to run this code but it's showing ValidationError. Here is my code

from langchain_google_genai import GoogleGenerativeAIEmbeddings

google_generative_ai_Embeddings =  GoogleGenerativeAIEmbeddings(model="models/embedding-001", google_api_key=api_key)
from langchain_community.vectorstores import Chroma
vectordb = Chroma.from_documents(data,
                           embedding=google_generative_ai_Embeddings,
                           persist_directory='./chromadb')
retriever_google = vectordb.as_retriever(score_threshold = 0.7)

from langchain.prompts import PromptTemplate

prompt_template = """Given the following context and a question, generate an answer based on this context only.
In the answer try to provide as much text as possible from "response" section in the source document context without making much changes.
If the answer is not found in the context, kindly state "I don't know." Don't try to make up an answer.

CONTEXT: {context}

QUESTION: {question}"""


PROMPT = PromptTemplate(
    template=prompt_template, input_variables=["context", "question"]
)
chain_type_kwargs = {"prompt": PROMPT}


from langchain.chains import RetrievalQA

chain_type = "stuff"

chain = RetrievalQA.from_chain_type(llm=llm,
                            chain_type=chain_type,
                            retriever=retriever_google,
                            input_key="query",
                            return_source_documents=True,
                            chain_type_kwargs=chain_type_kwargs)

and I am receiving this error

ValidationError                           Traceback (most recent call last)
~\AppData\Local\Temp\ipykernel_4328\826232400.py in <module>
     20 chain_type = "stuff"
     21 
---> 22 chain = RetrievalQA.from_chain_type(llm=llm,
     23                             chain_type=chain_type,
     24                             retriever=retriever_google,

~\AppData\Roaming\Python\Python39\site-packages\langchain\chains\retrieval_qa\base.py in from_chain_type(cls, llm, chain_type, chain_type_kwargs, **kwargs)
     98         """Load chain from chain type."""
     99         _chain_type_kwargs = chain_type_kwargs or {}
--> 100         combine_documents_chain = load_qa_chain(
    101             llm, chain_type=chain_type, **_chain_type_kwargs
    102         )

~\AppData\Roaming\Python\Python39\site-packages\langchain\chains\question_answering\__init__.py in load_qa_chain(llm, chain_type, verbose, callback_manager, **kwargs)
    247             f"Should be one of {loader_mapping.keys()}"
    248         )
--> 249     return loader_mapping[chain_type](
    250         llm, verbose=verbose, callback_manager=callback_manager, **kwargs
    251     )

~\AppData\Roaming\Python\Python39\site-packages\langchain\chains\question_answering\__init__.py in _load_stuff_chain(llm, prompt, document_variable_name, verbose, callback_manager, callbacks, **kwargs)
     71 ) -> StuffDocumentsChain:
     72     _prompt = prompt or stuff_prompt.PROMPT_SELECTOR.get_prompt(llm)
---> 73     llm_chain = LLMChain(
     74         llm=llm,
     75         prompt=_prompt,

~\AppData\Roaming\Python\Python39\site-packages\langchain\load\serializable.py in __init__(self, **kwargs)
     73 
     74     def __init__(self, **kwargs: Any) -> None:
---> 75         super().__init__(**kwargs)
     76         self._lc_kwargs = kwargs
     77 

~\AppData\Roaming\Python\Python39\site-packages\pydantic\v1\main.py in __init__(__pydantic_self__, **data)
    339         values, fields_set, validation_error = validate_model(__pydantic_self__.__class__, data)
    340         if validation_error:
--> 341             raise validation_error
    342         try:
    343             object_setattr(__pydantic_self__, '__dict__', values)

ValidationError: 1 validation error for LLMChain
llm
  Can't instantiate abstract class BaseLanguageModel with abstract methods agenerate_prompt, apredict, apredict_messages, generate_prompt, invoke, predict, predict_messages (type=type_error)

I was get this code from online source.And now I don't understand how to fix this issue? please help me to solve this issue.