Have you looked into the faq yet? It will give you some general answers on how to use the site as a creator: https://itch.io/docs/creators/faq
In regards to your game. Did you follow the process of making your game ready to publish? For example, some engines need to deploy the game beforehand if they have to be downloadable. I'm not sure what exactly you have to do step by step for Gamemaker, as I assume it depends on what platform you are targeting, among other things? You will probably find that out (if still needed), by taking a look into their dedicated faq or community support online.
In regards to uploading on itch.io, here a few steps to consider:
- Have you verified your e-mail adress already? That may block you from certain features of the site
- Where do you try to upload it exactly? Is it on your own created page?
- Which platform do you intend to publish for? There may be a chance that if you do not upload something that is targeted to run on users hardware, it may not work
- How large is your project you want to upload? You can by default only files with a max. size of 1gb (2gb if you use the butler tool)?
- You can also try to only keep the corresponding browser tab open and deactivate your antivirus at your own risk (maybe something is blocking the upload)
I am aware some of this advice is a bit general, but it may be helpful to exclude the most common things when you have just started out, before you can see if there is an specific problem with your case. I am also not 100% sure on some of the things I've written. For example, it could very well be that you should be able to upload your game at least into some category you can choose when uploading, which would hint at there being a more general issue blocking you. But I am not sure there.
Maybe someone else has some additional ideas to check here, as well.
