In response to a request I uploaded a text but I am unable to share it privately. When the public and private options appear only the public option allows me to click on it.
In case that you unwittingly uploaded the file as public one, see https://help.researchgate.net/hc/en-us/articles/14293099743121-How-to-make-content-private-or-remove-it.
Any Private file(s) uploaded on ResearchGate can be accessed by the Author or Co-author(s) only.
In the instant case, the request can be fulfilled by sending a massage to the concerned person and attaching the requested file, stored on your your computer, with the message.