Here is the release notes for buile 187: https://www.scirra.com/construct2/releases/r187
Main point of interest:
[quote:2mzzqiwl]Please note unfortunately this change has broken compatibility with the Scirra Arcade. The Arcade has long been in need of an update and we are planning to release a new version of it soon. When the new version is released Construct 2 will be updated to enable the Scirra Arcade exporter again, but unfortunately until then new submissions cannot be made from any version other than r168.2.
The reference to 168.2 is a mistake (no such build) and actually refers to build 186.2.
Therefore to post to the arcade, temporarily install r186.2 and export. (This will only work if you haven't used any of the new functionality introduced in subsequent builds.
The other option is to wait for the new Acrde to arrive, which considering how long it's taken, could be a long wait.