This form gave me an error:

#eb init --platform php7.0 --region us-east-1
ERROR: NotFoundError - The EB CLI cannot find a platform for key "php7.0".

This form worked:
#eb init --platform php-7.0 --region us-east-1
Application drupal-beanstalk has been created.

--platform php-7.0 (instead of php7.0) is the correct syntax per https://docs.aws.amazon.com/elasticbeanstalk/latest/dg/eb3-init.html

Thanks!