i installed 4.0.0-a2-dev from main branch on a database with pre-existing tables and got this error. 

Unknown database type enum requested, phpbb\db\middleware\mysql\platform may not support it.    
#0 [ROOT]/vendor/doctrine/dbal/src/Schema/MySQLSchemaManager.php(186): Doctrine/DBAL/Platforms/AbstractPlatform->getDoctrineTypeMapping('enum') 
#1 [ROOT]/vendor/doctrine/dbal/src/Schema/AbstractSchemaManager.php(1408): Doctrine/DBAL/Schema/MySQLSchemaManager->_getPortableTableColumnDefinition(Array)
#2 [ROOT]/vendor/doctrine/dbal/src/Schema/AbstractSchemaManager.php(423): Doctrine/DBAL/Schema/AbstractSchemaManager->_getPortableTableColumnList('phpbb_users', 'c7ukcom69_avath...', Array)   
#3 [ROOT]/vendor/doctrine/dbal/src/Schema/MySQLSchemaManager.php(66): Doctrine/DBAL/Schema/AbstractSchemaManager->doListTables()    
#4 [ROOT]/vendor/doctrine/dbal/src/Schema/AbstractSchemaManager.php(1651): Doctrine/DBAL/Schema/MySQLSchemaManager->listTables()    
#5 [ROOT]/vendor/doctrine/dbal/src/Schema/AbstractSchemaManager.php(1663): Doctrine/DBAL/Schema/AbstractSchemaManager->createSchema()   
#6 [ROOT]/phpbb/db/tools/doctrine.php(99): Doctrine/DBAL/Schema/AbstractSchemaManager->introspectSchema() 
#7 [ROOT]/phpbb/db/tools/doctrine.php(518): phpbb/db/tools/doctrine->get_schema() 
#8 [ROOT]/phpbb/db/tools/doctrine.php(218): phpbb/db/tools/doctrine->alter_schema(Object(Closure))    
#9 [ROOT]/phpbb/install/module/install_database/task/add_tables.php(134): phpbb/db/tools/doctrine->sql_create_table('bbdkp_register', Array)  
#10 [ROOT]/phpbb/install/sequential_task.php(58): phpbb/install/module/install_database/task/add_tables->execute_step('bbdkp_register', Array)
#11 [ROOT]/phpbb/install/module/install_database/task/add_tables.php(124): phpbb/install/module/install_database/task/add_tables->execute(Object(phpbb/install/helper/config), Array)     
#12 [ROOT]/phpbb/install/module_base.php(150): phpbb/install/module/install_database/task/add_tables->run()     
#13 [ROOT]/phpbb/install/installer.php(220): phpbb/install/module_base->run()     
#14 [ROOT]/phpbb/install/controller/install.php(136): phpbb/install/installer->run()
#15 [ROOT]/vendor/symfony/http-kernel/HttpKernel.php(101): phpbb/install/controller/install->phpbb/install/controller/{closure}()   
#16 [ROOT]/vendor/symfony/http-foundation/StreamedResponse.php(125): Symfony/Component/HttpKernel/HttpKernel::Symfony/Component/HttpKernel/{closure}()
#17 [ROOT]/vendor/symfony/http-foundation/Response.php(403): Symfony/Component/HttpFoundation/StreamedResponse->sendContent() 
#18 [ROOT]/install/index.php(62): Symfony/Component/HttpFoundation/Response->send() 
#19 {main}  

General Error:SQL ERROR [ mysqli ]Table 'c7ukcom69_avathar_be.phpbb4_config' doesn't exist [1146]SQLSELECT config_name, config_value, is_dynamic FROM phpbb4_config in file /customers/f/4/d/c7ukcom69/webroots/www/forum4/phpbb/db/driver/driver.php on line 1094    

FILE: (not given by php)
LINE: (not given by php)
CALL: installer_msg_handler() 

FILE: [ROOT]/phpbb/db/driver/driver.php   
LINE: 1094  
CALL: trigger_error() 

FILE: [ROOT]/phpbb/db/driver/mysqli.php   
LINE: 212   
CALL: phpbb\db\driver\driver->sql_error() 

FILE: [ROOT]/phpbb/db/driver/factory.php  
LINE: 355   
CALL: phpbb\db\driver\mysqli->sql_query() 

FILE: [ROOT]/phpbb/config/db.php    
LINE: 86    
CALL: phpbb\db\driver\factory->sql_query()

FILE: [ROOT]/phpbb/config/db.php    
LINE: 55    
CALL: phpbb\config\db->initialise() 

FILE: (not given by php)
LINE: (not given by php)
CALL: phpbb\config\db->__construct()

FILE: [ROOT]/vendor/symfony/dependency-injection/ContainerBuilder.php   
LINE: 1197  
CALL: ReflectionClass->newInstanceArgs()  

FILE: [ROOT]/vendor/symfony/dependency-injection/ContainerBuilder.php   
LINE: 632   
CALL: Symfony\Component\DependencyInjection\ContainerBuilder->createService()     

FILE: [ROOT]/vendor/symfony/dependency-injection/ContainerBuilder.php   
LINE: 577   
CALL: Symfony\Component\DependencyInjection\ContainerBuilder->doGet()   

FILE: [ROOT]/phpbb/di/container_builder.php   
LINE: 454   
CALL: Symfony\Component\DependencyInjection\ContainerBuilder->get()     

FILE: [ROOT]/phpbb/di/container_builder.php   
LINE: 163   
CALL: phpbb\di\container_builder->load_extensions() 

FILE: [ROOT]/phpbb/install/helper/container_factory.php     
LINE: 160   
CALL: phpbb\di\container_builder->get_container()   

FILE: [ROOT]/phpbb/install/helper/container_factory.php     
LINE: 91    
CALL: phpbb\install\helper\container_factory->build_container()   

FILE: [ROOT]/phpbb/install/installer.php  
LINE: 318   
CALL: phpbb\install\helper\container_factory->get() 

FILE: [ROOT]/phpbb/install/controller/install.php   
LINE: 136   
CALL: phpbb\install\installer->run()

FILE: [ROOT]/vendor/symfony/http-kernel/HttpKernel.php
LINE: 101   
CALL: phpbb\install\controller\install->phpbb\install\controller

{closure}()

FILE: [ROOT]/vendor/symfony/http-foundation/StreamedResponse.php  
LINE: 125   
CALL: Symfony\Component\HttpKernel\HttpKernel::Symfony\Component\HttpKernel
{closure}

()   

FILE: [ROOT]/vendor/symfony/http-foundation/Response.php    
LINE: 403   
CALL: Symfony\Component\HttpFoundation\StreamedResponse->sendContent()  

FILE: [ROOT]/install/index.php
LINE: 62    
CALL: Symfony\Component\HttpFoundation\Response->send()