How to create an uploads subfolder on a multisite setup?

It seems like you are creating the folder on main site and trying to read from it on a sub site. This is not the right way to go, remeber that on multisite each site, except for user management, is almost 100% isolated full wordpress that has no easy access to data from other sites.

Assuming there should be only one such file in the system, What you probably need to do is to do the file creation and access in the context of the main site which is usually blog #1, something like

// to write
swicth_to_blog(1);
write file data
restore_blog();

// to read
swicth_to_blog(1);
read file data
restore_blog();