After migration from magento 1.9 to magento 2.3.3, I am not able to edit customer from the admin and getting following errors (https://prnt.sc/qx7hvl).
That issue occurred because your database had at least one custom attribute which had the frontend_input as ‘select/multiselect’, however the related ‘source_model’ were missing in M2.
You can run the following SQL in your M2 database to look for that attribute at your end:
Select * From eav_attribute Where frontend_input IN ('select', 'multiselect') AND is_user_defined = 1 AND (`source_model` = '' OR `source_model` IS NULL) AND attribute_code NOT IN ('manufacturer', 'color');
And then, you need to change the value of ‘frontend_input’ field in the found data records from ‘select’ or ‘multiselect’ to ‘text’.
Once done, clean M2 cache and check the issue you mentioned again.