Being a retro-styled platformer, most people assumed that when Sonic Mania launched on PC you'd be able to play it without an internet connection. It was an assumption, sure, but many would say a reasonable one: after all, you can play offline on consoles and Switch, so why not on your laptop? Well, it turns out the Steam version of Sonic Mania makes use of the much-maligned Denuvo DRM. As such, it will not boot up unless you're connected to the internet.
The game will run properly if you connect to the internet, boot the game, then keep the game running while you leave internet range, if you're especially desperate. But if you're offline on Steam you won't be able to open Sonic Mania.
Interestingly enough it seems like some of those working on the game are not fans of Denuvo either. Sonic the Hedgehog PR manager Aaron Webber asked fans to let their voices be heard regarding DRM, implying he was not a fan of the implementation.
In particular, please do share your feedback on DRM or any issues you're having at the link above. Make your voices heard.
— Aaron Webber (@RubyEclipse) August 29, 2017
While the Steam page for Sonic Mania did not mention Denuvo or an online connection requirement when the game first released, the page has since been updated.