You need to ensure that Scan logging is enabled. Scans will only get logged after you enabled it. You won't be able to list scan attempts which were made before enabling scan logging.
Alternatively run the following report which lists the last scan dates for all your assets:
Select Top 1000000 tblAssets.AssetID,
tblAssets.AssetName,
tblAssets.IPAddress,
tblAssets.Lastseen,
tblAssets.Lasttried,
tblAssets.Lasttriggered,
tblAssets.LastScheduled,
tblAssets.LastActiveScan,
tblAssets.LastIPScan,
tblAssets.LastLsPush
From tblAssets
Order By tblAssets.Lasttried Desc