Hello,
I’m sorry that you’re running into this issue, but I’ll be happy to help you figure out what’s going on.
Some browsers (such as Safari) automatically unzip files when they are downloaded. If the file you have is not zipped, then you will need to zip it manually before uploading (all plugin files need to be zipped to be uploaded through the WordPress Dashboard). Alternatively, you can upload the plugin manually by following these instructions. Can you give this a try and let me know if it works for you?
If not, please make sure that you are running the current version of WordPress (4.9.5)? Also, is this a multi-site install?