That is the result when I try to install an plugin
Problem 1
- The requested package phpspec/prophecy (locked at 1.4.x-dev) exists as phpspec/prophecy[1.1.2, 1.4.0, dev-master, 1.6.x-dev, v1.0.0, v1.0.0-BETA1, v1.0.0-BETA2, v1.0.1, v1.0.2, v1.0.3, v1.0.4, v1.1.0, v1.1.1, v1.2.0, v1.2.1, v1.3.0, v1.3.1, v1.4.1, v1.5.0, v1.6.0, v1.6.1] but these are rejected by your constraint.
Problem 2
- The requested package sebastian/comparator (locked at 1.1.x-dev) exists as sebastian/comparator[1.0.0, 1.0.1, 1.1.0, 1.1.1, 1.1.2, 1.2.0, dev-master, 1.2.x-dev] but these are rejected by your constraint.
Problem 3
- The requested package sebastian/diff (locked at 1.3.x-dev) exists as sebastian/diff[1.0.0, 1.1.0, 1.2.0, 1.3.0, 1.4.0, 1.4.1, dev-master, 1.4.x-dev] but these are rejected by your constraint.
Problem 4
- The requested package sebastian/exporter (locked at 1.2.x-dev) exists as sebastian/exporter[1.0.0, 1.0.1, 1.0.2, 1.1.0, 1.2.0, 1.2.1, 1.2.2, dev-factory, dev-master, 1.3.x-dev] but these are rejected by your constraint.
Problem 5
- Installation request for phpunit/phpunit (locked at 4.8.x-dev, required as ~4.0) -> satisfiable by phpunit/phpunit[4.8.x-dev].
Try backing up everything, and then deleting composer.lock in your main newscoop directory, before installing the plugin again.
(A new composer.lock file should be created when the plugin is installed - you will find the problem arises because the standard composer.lock file specifies development instead of production versions of files).