I believe the problem is rooted in the passing of the credential flow variable to the Vector Store Retriever node in the RAG ChatApp component.
Go inside the RAG ChatApp component and make sure to include the credential variable (see screenshot). This is needed because the downstream Vector Store Retriever node in the component requires OpenAI embeddings, and therefore the connection to OpenAI service (via credential flow variable) needs to persist.
Additionally, be aware that if the RAG ChatApp contains the Chat Model Prompter node, you need to use the OpenAI Chat Model Connector node (and not the OpenAI LLM Connector node). This is because you need a conversational LLM for chatting.