i encountered the same error and did not found a google solution:
2019-05-06T10:19:10 [D] Providers ['dhcp_isc'] are going to be configured for 'dhcp'
2019-05-06T10:19:10 [D] 'dhcp_isc' settings: 'blacklist_duration_minutes': 1800 (default), 'config': /etc/dhcp/dhcpd.conf (default), 'dhcp_config': /etc/dhcp/dhcpd.conf, 'dhcp_key_name': dhcp-lbsrv-omapi, 'dhcp_key_secret': BbPY9YZO1nCzD
T3iTzks84n/fpupew5WCjVaaKVC4jTji6Gc6DvqAnjXDMSIECAImo43BpZV+h20YHH+BQe2rA==, 'dhcp_leases': /var/lib/dhcp/dhcpd.leases, 'dhcp_vendor': isc, 'leases': /var/lib/dhcp/dhcpd.leases, 'leases_file_observer': inotify_leases_file_observer, 'omapi
_port': 7911 (default), 'server': 127.0.0.1, 'subnets': [], 'use_provider': dhcp_isc
2019-05-06T10:19:10 [I] Successfully initialized 'foreman_proxy'
2019-05-06T10:19:10 [E] Couldn't enable 'dhcp_isc': cannot load such file -- rb-inotify
2019-05-06T10:19:10 [D] <Array> ["/usr/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require'", "/usr/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require'", "/usr/share/foreman-proxy/modules/dhcp_isc/inotify_lea
ses_file_observer.rb:1:in `<top (required)>'", "/usr/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require'", "/usr/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:59:in `require'", "/usr/share/foreman-proxy/modules/dhcp_is
c/configuration_loader.rb:31:in `load_dependency_injection_wirings'", "/usr/share/foreman-proxy/lib/proxy/plugin_initializer.rb:231:in `block in wire_up_dependencies'", "/usr/share/foreman-proxy/lib/proxy/plugin_initializer.rb:230:in `eac
h'", "/usr/share/foreman-proxy/lib/proxy/plugin_initializer.rb:230:in `wire_up_dependencies'", "/usr/share/foreman-proxy/lib/proxy/plugin_initializer.rb:220:in `configure_plugin'", "/usr/share/foreman-proxy/lib/proxy/plugin_initializer.rb
:92:in `block in configure'", "/usr/share/foreman-proxy/lib/proxy/plugin_initializer.rb:92:in `each'", "/usr/share/foreman-proxy/lib/proxy/plugin_initializer.rb:92:in `configure'", "/usr/share/foreman-proxy/lib/proxy/plugin_initializer.rb
:170:in `block in initialize_plugins'", "/usr/share/foreman-proxy/lib/proxy/plugin_initializer.rb:170:in `each'", "/usr/share/foreman-proxy/lib/proxy/plugin_initializer.rb:170:in `initialize_plugins'", "/usr/share/foreman-proxy/lib/launcher.rb:168:in `launch'", "/usr/share/foreman-proxy/bin/smart-proxy:6:in `<main>'"]
2019-05-06T10:19:10 [E] Error during startup, terminating. Dependency 'leases_observer' is undefined
2019-05-06T10:19:10 [D] <Array> ["/usr/share/foreman-proxy/lib/proxy/dependency_injection.rb:58:in `get_dependency'", "/usr/share/foreman-proxy/lib/proxy/plugin_initializer.rb:113:in `block (2 levels) in stop_services'", "/usr/share/foreman-proxy/lib/proxy/plugin_initializer.rb:113:in `map'", "/usr/share/foreman-proxy/lib/proxy/plugin_initializer.rb:113:in `block in stop_services'", "/usr/share/foreman-proxy/lib/proxy/plugin_initializer.rb:112:in `each'", "/usr/share/for
eman-proxy/lib/proxy/plugin_initializer.rb:112:in `stop_services'", "/usr/share/foreman-proxy/lib/proxy/plugin_initializer.rb:95:in `rescue in configure'", "/usr/share/foreman-proxy/lib/proxy/plugin_initializer.rb:90:in `configure'", "/us
r/share/foreman-proxy/lib/proxy/plugin_initializer.rb:170:in `block in initialize_plugins'", "/usr/share/foreman-proxy/lib/proxy/plugin_initializer.rb:170:in `each'", "/usr/share/foreman-proxy/lib/proxy/plugin_initializer.rb:170:in `initi
alize_plugins'", "/usr/share/foreman-proxy/lib/launcher.rb:168:in `launch'", "/usr/share/foreman-proxy/bin/smart-proxy:6:in `<main>'"]
i was able to fix it with installing this package:
apt install ruby-rb-inotify