Cannot create release for my contributed module
There is just a small change that you need to make. The projects on drupal.org are expected to have the releases in lowercase. This is done to support the project infrastructure which is in place like update status and the testing bot.

You should be able to create a new release with a tag like 8.x-1.0-rc1

More info here : https://www.drupal.org/node/1015226