This is very strange. The output of sudo apt install --dry-run playonlinux should be showing exactly what would happen if you actually installed playonlinux through apt which would indicate that it is in the 17.04 repositories.
Searching for playonlinux in apt yeilds:
steven@MacBook51:~$ apt-cache showpkg playonlinux
Package: playonlinux
Versions:
4.2.10-2ubuntu1 (/var/lib/apt/lists/us.archive.ubuntu.com_ubuntu_dists_zesty_multiverse_binary-amd64_Packages) (/var/lib/apt/lists/us.archive.ubuntu.com_ubuntu_dists_zesty_multiverse_binary-i386_Packages)
Description Language:
File: /var/lib/apt/lists/us.archive.ubuntu.com_ubuntu_dists_zesty_multiverse_binary-amd64_Packages
MD5: 729a863a5c2d8eb304d4e8e17ffbeafe
Description Language: en
File: /var/lib/apt/lists/us.archive.ubuntu.com_ubuntu_dists_zesty_multiverse_i18n_Translation-en
MD5: 729a863a5c2d8eb304d4e8e17ffbeafe
Reverse Depends:
wine-development,playonlinux
wine-stable,playonlinux
Dependencies:
4.2.10-2ubuntu1 - python:any (2 2.7.5-5~) wine (16 (null)) wine-development (0 (null)) unzip (0 (null)) wget (0 (null)) xterm (16 (null)) x-terminal-emulator (0 (null)) python-wxgtk3.0 (0 (null)) imagemagick (0 (null)) cabextract (0 (null)) mesa-utils (0 (null)) gettext-base (0 (null)) binutils (0 (null)) gnupg (0 (null)) icoutils (0 (null)) x11-utils (0 (null)) curl (0 (null)) netcat (0 (null)) bzip2 (0 (null)) p7zip-full (0 (null)) ttf-mscorefonts-installer (0 (null)) scrot (0 (null))
Provides:
4.2.10-2ubuntu1 -
Reverse Provides:
steven@MacBook51:~$
Which repository are you using?