When trying to Test Credential via the Credential page, an error is thrown "This credential type cannot be tested", even though you know from the documentation that this is a type that this feature supports.
This will usually happen with every credential, even new ones - however this does not mean that the credential is actually failing and does not work.
The cause could be due to a customisation of the Display Business Rule "Add credential data to scratchpad" that populates the scratchpad data when the credential form loads, or the UI Action "Test Credential" which takes that scratchpad data and uses it in the ecc queue payload for the MID Server probe that actually tests the credential.
An old customised version of the business rule, which did not include the sys_id in the data, has been seen to cause this issue.
The resolution for this issue is to check the version of these 2 records are the latest System Upgrades version, and not a potentially old and customised version:
If these were customised, upon upgrading an instance, these will be skipped for upgrade due to customisation, and therefor will not receive the updates it need.