Hi there, @palmanoro.
Thanks for reaching out to us here in the Community. I'm here to help make sure that you can e-file W-2s in QuickBooks Online (QBO).
Just to confirm, are you using Intuit Online Payroll (IOP)? If so, in-product support for federal forms is available only with an Enhanced level subscription. Additionally, make sure that you already applied and enabled e-file or eService.
If you already did that and the option to e-file is still grayed out, let's perform some troubleshooting steps that can help us isolate this. Try logging into your QBO account using a different or a private browser. There are times that the browser is full of frequently-accessed page resources, thus causing some errors and/or unusual responses.
To use a private browser, here's how:
Once logged in, process and file W-2 forms again to double-check. If it'll work, go back to your default browser and perform clear cache to remove temporary internet files on your computer. Before doing so, make sure to take note your saved passwords and URLs. Once done, here's a helpful article with the steps on how to perform clear cache: How do I clear my browser cache and temporary Internet files.
That's it! Give this a try and let me know how it goes in the comment section below. Please know that I'm only a post away if you have any follow-up questions. Cheers!