我最近在 Ubuntu 18.04 服务器上安装了 Anaconda，今天，当我安装一些新软件包时，出现了一个警告：
==> WARNING: A newer version of conda exists. <==
current version: 4.6.14
latest version: 4.7.5
Please update conda by running
$ conda update -n base -c defaults conda
因此，我执行了上述命令（r_env如果重要的话，在我的环境中），一切似乎都很好：
(r_env) user@server:~$ conda update -n base -c defaults conda
Collecting package metadata: done
Solving environment: done
## Package Plan ##
environment location: /home/user/anaconda3
added / updated specs:
- conda
The following packages will be downloaded:
package | build
---------------------------|-----------------
conda-4.7.5 | py37_0 3.0 MB
conda-package-handling-1.3.10| py37_0 259 KB
------------------------------------------------------------
Total: 3.2 MB
The following NEW packages will be INSTALLED:
conda-package-han~ pkgs/main/linux-64::conda-package-handling-1.3.10-py37_0
The following packages will be UPDATED:
ca-certificates 2019.1.23-0 --> 2019.5.15-0
certifi 2019.3.9-py37_0 --> 2019.6.16-py37_0
conda 4.6.14-py37_0 --> 4.7.5-py37_0
openssl 1.1.1b-h7b6447c_1 --> 1.1.1c-h7b6447c_1
The following packages will be DOWNGRADED:
anaconda 2019.03-py37_0 --> custom-py37_0
Proceed ([y]/n)? y
但是，此后我无法安装新软件包。它显示：
Collecting package metadata (current_repodata.json): done
Solving environment: failed
Collecting package metadata (repodata.json): done
Solving environment: failed
PackagesNotFoundError: The following packages are not available from current channels:
- pkgs/main/linux-64::r-readxl==1.3.1=r36h29659fb_0
- pkgs/main/linux-64::r-stringi==1.4.3=r36h29659fb_0
< ...dozen of r-packages...>
- pkgs/main/linux-64::r-tidyr==0.8.3=r36h29659fb_0
- pkgs/main/noarch::r-reprex==0.2.1=r36h6115d3f_0
Current channels:
- https://repo.anaconda.com/pkgs/main/linux-64
- https://repo.anaconda.com/pkgs/main/noarch
- https://repo.anaconda.com/pkgs/r/linux-64
- https://repo.anaconda.com/pkgs/r/noarch
To search for alternate channels that may provide the conda package you're
looking for, navigate to
https://anaconda.org
and use the search bar at the top of the page.
我尝试降级conda到 4.6.14，但没有成功 - 出现相同的错误。我可以在不重新安装 Anaconda 的情况下修复该问题吗？