查看软件包版本但无法安装

这是我迄今为止尝试过的

$ apt-cache policy couchdb
couchdb:
  Installed: 3.0.0~bionic
  Candidate: 3.0.0~bionic
  Version table:
 *** 3.0.0~bionic 500
        500 https://apache.bintray.com/couchdb-deb bionic/main amd64 Packages
        100 /var/lib/dpkg/status
     2.3.1~bionic 500
        500 https://apache.bintray.com/couchdb-deb bionic/main amd64 Packages
     2.3.0~bionic 500
        500 https://apache.bintray.com/couchdb-deb bionic/main amd64 Packages
     2.2.0~bionic 500
        500 https://apache.bintray.com/couchdb-deb bionic/main amd64 Packages
     2.1.2~bionic 500
        500 https://apache.bintray.com/couchdb-deb bionic/main amd64 Packages

我想安装 2.3.1 版，所以我继续：

$ sudo apt-get install couchdb=2.3.1
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Version '2.3.1' for 'couchdb' was not found

我在这里错过了什么？