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Sorry to hear you’re facing problems 
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In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 20.0.5): 20
Operating system and version (eg, Ubuntu 20.04): official docker container nextcloud:20
The issue you are facing:
Is this the first time you’ve seen this error? (Y/N): Y
Steps to replicate it:
-
Create a new database in Postgres
-
Attempt to migrate from SQLite to Postgres via
php occ db:convert-type --password=“e…q” pgsql nextcloud postgres nextcloud
The output of your Nextcloud log in Admin > Logging the prompt within the docker container:
www-data@f4145bc6612f:~/html$ php occ db:convert-type --password="e..q" pgsql nextcloud postgres nextcloud
Creating schema in new database
In AbstractPostgreSQLDriver.php line 79:
An exception occurred while executing 'DROP INDEX "primary"':
SQLSTATE[42704]: Undefined object: 7 ERROR: index "primary" does not exist
In PDOConnection.php line 83:
SQLSTATE[42704]: Undefined object: 7 ERROR: index "primary" does not exist
In PDOConnection.php line 78:
SQLSTATE[42704]: Undefined object: 7 ERROR: index "primary" does not exist
db:convert-type [--port PORT] [--password PASSWORD] [--clear-schema] [--all-apps] [--chunk-size CHUNK-SIZE] [--] <type> <username> <hostname> <database>
www-data@f4145bc6612f:~/html$
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'htaccess.RewriteBase' => '/',
'memcache.local' => '\\OC\\Memcache\\APCu',
'apps_paths' =>
array (
0 =>
array (
'path' => '/var/www/html/apps',
'url' => '/apps',
'writable' => false,
),
1 =>
array (
'path' => '/var/www/html/custom_apps',
'url' => '/custom_apps',
'writable' => true,
),
),
'instanceid' => 'o....y',
'passwordsalt' => 'Y...x',
'secret' => 'T...b',
'trusted_domains' =>
array (
0 => 'nextcloud.MY.DOMAIN',
),
'datadirectory' => '/var/www/html/data',
'dbtype' => 'sqlite3',
'version' => '20.0.5.2',
'overwrite.cli.url' => 'https://nextcloud.MY.DOMAIN',
'overwriteprotocol' => 'https',
'auth.bruteforce.protection.enabled' => false,
'installed' => true,
'has_rebuilt_cache' => true,
'maintenance' => false,
'theme' => '',
'loglevel' => 2,
'mail_from_address' => 'nextcloud',
'mail_smtpmode' => 'smtp',
'mail_sendmailmode' => 'smtp',
'mail_domain' => 'swtk.info',
'mail_smtpauthtype' => 'LOGIN',
'mail_smtpauth' => 1,
'mail_smtphost' => 'smtp.orange.fr',
'mail_smtpsecure' => 'ssl',
'mail_smtpport' => '465',
'mail_smtpname' => 'XXX@orange.fr',
'mail_smtppassword' => 'XXX',
'has_internet_connection' => true,
'twofactor_enforced' => 'false',
'twofactor_enforced_groups' =>
array (
),
'twofactor_enforced_excluded_groups' =>
array (
),
'filelocking.enabled' => true,
'trusted_proxies' =>
array (
0 => '172.19.0.31',
),
'forwarded_for_headers' =>
array (
0 => 'HTTP_X_FORWARDED_FOR',
),
'app_install_overwrite' =>
array (
0 => 'mindmap_app',
1 => 'twofactor_yubikey',
),
);
NOTE: I use double a multiple factor authentication, I saw some old (3+ years) discussions about issues with that when migrating (not necessarily from SQLite)