There are a few potential issues, mostly commonly the following.
When you first sign up for Heroku you might not have provided any billing information (i.e. credit card). Heroku will not allow you to scale if this is the case. Be sure to add your credit card and you should be good to go.
When this error (UnprocessableEntity) is returned by Heroku (and displayed in the HireFire UI) it's often the case that you're trying to scale beyond the Heroku-imposed scaling constraints.
To quote from Heroku's official documentation:
hobby dyno types only support a maximum of one dyno running per process type. Additionally, applications using a
free dyno type are limited to a maximum of two concurrent running dynos.
By default, all applications are limited to 100 dynos. Additionally, a process type can’t be scaled to more than 10 dynos for
Submit a request to raise this limit for your application."