Your issue is not code related and therefore should really be directed to Wix Support instead.
https://support.wix.com/en/article/contacting-wix-support
As for the upload button itself, go here to read this support page.
https://support.wix.com/en/article/adding-and-setting-up-an-upload-button
Also, you can do a simple code example like this one to work with your upload button.
https://support.wix.com/en/article/wix-code-using-the-upload-button-with-code
For more info on the upload button through code, look at the Wix API reference section here.
https://www.wix.com/corvid/reference/$w.UploadButton.html