The user_data key is a property of the OS::Nova::Server resource...so anywhere you have an OS::Nova::Server, you can specify a user_data property. The error you are getting suggests that your nested template has a parameters section with a user_data property that has no default (making it a required property). If that's the case, you can simply remote it from your parameters section if you don't need it.
This repository has some examples that do exactly what you're trying to do -- provide a user_data property to an OS::Nova::Server resource that is part of a nested stack included from another stack. In particularly, look at wp-ssacling-lb.yaml and the nested template, wp-nested-server-lb.yaml.
The user_data key is a property of the OS::Nova::Server resource...so anywhere you have an OS::Nova::Server, you can specify a user_data property. The error you are getting suggests that your nested template has a parameters section with a user_data property that has no default (making it a required property). If that's the case, you can simply remote it from your parameters section if you don't need it.
This repository has some examples that do exactly what you're trying to do -- provide a user_data property to an OS::Nova::Server resource that is part of a nested stack included from another stack. In particularly, look at wp-ssacling-lb.yamlwp-scaling-lb.yaml and the nested template, wp-nested-server-lb.yaml.