Yes, this model (like many, especially older SSDs) does not have temperature sensor at all.
This is why the temperature value is unknown - as the SSD provides no temperature at all.
If you check the S.M.A.R.T. page of the SSD in Hard Disk Sentinel, you can see the special self monitoring attributes and their current status.
For this SSD, there is no "Temperauture" or similar listed there - confirming that the SSD has no actual temperature sensor.
In contrast, if you have any modern hard disk (or a different SSD) and check the S.M.A.R.T. page, you may see one (or even more) "Temperature" attribute(s). These are examined their values displayed by Hard Disk Sentinel as actual temperature for the hard disks / SSD.
This is missing from the SSD, the manufacturer did not add a such sensor.