This is because subscribers do not have permission to upload files. You will need to either assign them to a role that can upload files or you’ll need to alter the subscriber permissions to allow them to upload. This https://wordpress.org/plugins/user-role-editor/ or similar plugins will let you do that.
But, a word of warning. Allowing subscribers the ability to upload files can be considered a security vulnerability. I would not do this on a site where anyone can join the site. You might want to think about an alternative. For example, you could create a repeater field with image sub fields. Then you can use the repeater in place of the gallery. If you want to keep the gallery you could build either an acf/pre_save_post or acf/save_post filter to copy the images from the repeater into the gallery.
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