The reason why you're receiving that error is because Magento 2 uses json to store complex data within database, so the column "extra" of the "admin_user" table has a format miss-match.
If you've copied it from from Magento 1 to Magento 2 you need to ensure that you re-parse values of that column.
Magento 1 "admin.user"admin_user table extra column" stores serialized data.
Magento 2 "admin.user"admin_user table extra column" stores json encoded data.
That being said - there are many other issues that would need to be solved to achieve what you need. It is not only the admin_user table that you need to merge. You also need to merge roles and rules. To make matters worse - the Magento 2 ACL is quite a bit different than Magento 1.
On top of that - the Magento 1 and Magento 2 use different password algorithms, so just copy-pasting the column values will not work.
In general - unless you're migrating hundreds of admin accounts the best way to do it is what Magento guidelines recommend - recreate them manually reusing some of the data from Magento 1, such as name, username, email.
Reference: https://devdocs.magento.com/guides/v2.3/migration/migration-manually.html