The error looks like this:
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package mysql-workbench is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'mysql-workbench' has no installation candidate
How do I install it?
For the people not willing to wait, You could try using DBeaver. I can confirm that it works on Ubuntu 19.04. If you insist on Workbench, it is also possible to run the Windows version through wine (performance does not seem to be impacted).
It is also possible to install the ubuntu 18 package of mysql-workbench from https://dev.mysql.com/downloads/workbench/
To get this working you first have to fullfill the dependencies: Download libzip4 from https://packages.ubuntu.com/bionic/libzip4 and install it. The second dependency, which sadly I do not know anymore, you can solve by installing the package via apt.