In Zabbix API it is possible to create host with inventory, but if host is already created and inventory this host is disabled, it is not possible to update this host to create inventory.

in your API class CHost.php there is some additional code that restrict to create inventory while Host Update function.

Host update function actially call massUpdate function and here we see this code:

if (!isset($existingInventoriesDb[$hostId])) {
							$host = get_host_by_hostid($hostId);
							self::exception(ZBX_API_ERROR_PARAMETERS, _s(
								'Inventory disabled for host "%1$s".', $host['host']
							));
						}

Why you cant create new inventory for host if there is no inventory for host exists yet?