OS: Mac OS X 10.10.3
I had Basic Mac Tex Live 2014 before. I upgraded to Mac Tex Live 2015 and then uninstall Tex Live 2014 by removing the folder /usr/local/texlive/2014basic.
After the upgrade, I found the installed packages missing when compiling the tex file. I checked in the Tex Live Utility and found some additional packages are Not Installed (i.e. the packages that are not distributed with basicTex).
When I tried to install the packages, an error popped out:
The repository at http://mirror.ox.ac.uk/sites/ctan.org/systems/texlive/tlnet
has TeX Live 2015, but you have TeX Live 18446744073709551615 installed.
You need to switch repositories in order to continue.
I wonder where the version 18446744073709551615 come from. In the configuration of distribution, it is shown that TeXLive-2015-Basic is installed.
EDIT
Sorry about not searching carefully. I found the cause and a workaround at the GitHub of Tex Live Utility: Issue 17.
The issue seems to be due to the distribution of Mac TeX Live.
Best Answer
I found other people come across the same problem and find the workaround via my question. So I undelete the question.
Please refer to Issues page of TeX Live Utility: here for the cause and workaround.
It seems that a new release of MacTeX Live 2015 has fixed the problem.