It seems like the app server is down. You can't download the set-up file, If you do have it returns a timed out signal when it tries to reachout to the server to build and it gives a server error when you try to install things using it.
Viewing post in Itch.io App Download Error
Someone else was talking about some software being down and not allowing creators to submit installable files. This probably means there's some third party software down that itch relied on for installations. That also may be happening but idk my own experience suggests the app download server is also inaccessible as a result for some reason but that also may just be a weird cascading effect. Either way it's a server side issue. Not much can be done.