The plugin is not able to create jenkins slave. The jenkins logs show the following exception message (hidding some of my details)

com.myjeeva.digitalocean.exception.RequestUnsuccessfulException: DigitalOcean API request unsuccessful, possible reason colud be incorrect values https://api.digitalocean.com/droplets/new/?client_id=xxxxxxxxxx&api_key=xxxxxxxx&name=jenkins_ea8482b1-665c-4d5b-805d-a5bbe35c50c3&region_id=3&ssh_key_ids=xxx&image_id=xxx&size_id=63.

DigitalOcean support says that the plugin has invalid values for host name

As for the API call itself, it appears that your hostname in the call contains invalid characters:

{"status":"ERROR","error_message":"Name Only valid hostname characters are allowed. (a-z, A-Z, 0-9, . and -)","message":"Name Only valid hostname characters are allowed. (a-z, A-Z, 0-9, . and -)"}

Please try again using another hostname.