As discussed in pypa/pipenv#4900, Requirementslib has an issue when the given relative path and the working directory are different. For example, if the working directory is changed to something other than where the python script is executed, but the relative path is relative to where the script was executed, Requirementslib will fail resolving the requirement.
While it might make sense as to why it's impossible to do so, I wish that the error message was worded clearer and not so crypticly.