Do you have a pro or business account? I found that pro can use the connector with the mcp in Dev (you are) and chat gpt should be in dev mode too. If you are using a business account (like i do) You cannot use the standard tool and need to create your own tool (stupid AF)
Make sure you have the correct tools. If you have ChatGPT Business: Build your tools. ChatGPT Business supports only fetch and search tools, that are not included in the MCP Standard Tools SuiteApp, and you must build them yourself. For more information, see Creating Custom Tools for the NetSuite AI Connector Service. If you have ChatGPT Plus or Pro, you can use the MCP Standard tools SuiteApp, but you must first enable the Developer mode in ChatGPT. Before enabling Developer mode, review the Associated Risks, Controls, and Mitigation Strategies topic, as well as any OpenAI documentation about risks and safety of the MCP and the Developer mode. If you're confident to proceed, go to Setting > Connectors > Advanced > Developer mode.