In IBM UrbanCode Deploy, applications don't have versions. Components have versions.
If you've got the snapshot that has been deployed to the environment, you can get the component versions in that snapshot with this command:
https://www.ibm.com/support/knowledgecenter/SS4GSP_6.2.4/com.ibm.udeploy.api.doc/topics/rest_cli_snapshot_getsnapshotversions_get.html
If you don't have a snapshot, you can use the internal, unsupported API to get the current desired inventory. The command would be something like this:
GET https://ucdserver.example.com:8443/rest/deploy/environment/9e022848-ca4f-447e-9311-3d77103c612c/latestDesiredInventory/true?rowsPerPage=50&pageNumber=1&orderField=name&sortType=desc
That command returns JSON with the versions that have most recently been deployed to the environment and a lot of other info about the environment inventory.