Welcome to Microsoft Q&A!
You are not the only one who meets the issue these days. Currently, a working solution from the Developer Community of Visual Studio here: Failed to sign appx is that you could try to uninstall Windows Update (KB4535996) and reboot.
Another thing is that right now Microsoft Q&A is under public preview, all the topics that supported now are listed in the right top. For questions about publishing UWP apps, you could ask in Submitting and Publishing Apps Forum in MSDN.