This will cause if you accidentally delete default attribute set in that case.
1.Truncate the following table eav_attribute_group,eav_attribute_set(Disable Foreign key constrain)
2.Insert the table
INSERT INTO eav_attribute_set
(attribute_set_id
, entity_type_id
,
attribute_set_name
, sort_order
) VALUES (1, 1, 'Default', 2),(2, 2,
'Default', 2),(3, 3, 'Default', 1),(4, 4, 'Default', 1),(5, 5,
'Default', 1),(6, 6,'Default', 1),(7, 7, 'Default', 1),(8, 8,
'Default', 1),(10, 10, 'Default', 1);
INSERT INTO eav_attribute_group
(attribute_group_id
,
attribute_set_id
, attribute_group_name
, sort_order
,
default_id
, attribute_group_code
, tab_group_code
) VALUES (1, 1,
'General', 1, 1, 'general', NULL),(4, 2, 'General', 1, 1, 'general',
NULL),(7, 3, 'General', 10, 1, 'general', NULL),(10, 3, 'General
Information', 2, 0,'general-information', NULL),(13, 3, 'Display
Settings', 20, 0, 'display-settings', NULL),(16, 3, 'Custom Design',
30, 0, 'custom-design', NULL),(19, 4, 'Product Details', 10, 1,
'product-details', 'basic'),(22, 4, 'Advanced Pricing', 40, 0,
'advanced-pricing', 'advanced'),(25, 4, 'Search Engine Optimization',
30, 0, 'search-engine-optimization', 'basic'),(28, 4, 'Images', 20, 0,
'image-management', 'basic'),(31, 4, 'Design', 50, 0, 'design',
'advanced'),(34, 4, 'Autosettings', 60, 0, 'autosettings',
'advanced'),(37, 4, 'Content', 15, 0, 'content', 'basic'),(40, 4,
'Schedule Design Update',55, 0, 'schedule-design-update',
'advanced'),(43, 4, 'Bundle Items', 16, 0, 'bundle-items', NULL),(46,
5, 'General', 1, 1, 'general', NULL),(49, 6, 'General', 1, 1,
'general', NULL),(52, 7, 'General', 1, 1, 'general', NULL),(55, 8,
'General', 1, 1, 'general', NULL),(58, 4, 'Gift Options', 61, 0,
'gift-options', NULL),(61, 10, 'General', 1, 1, 'general', NULL);
It happens may be you have migrated data over existing data in it, please try migrate data with fresh database with each try. Even if the migration failed once you have to start migrate in fresh database.
Once the migration is failed then you have to fix and start migration from new database. Just follow below steps to make it easier.