I think your on the right track. Most likely you didn't join the AD table with the other correctly, since it isn't done automatically in the report builder.
You'll need to manually connect tbladusers to tblassets on both username and userdomain. Then you can enable the UPN and it should work.
This should be your result:
Select Top 1000000 tsysOS.Image As icon,
tblAssets.AssetID,
tblAssets.AssetName,
tblAssets.Domain,
tblAssets.Username,
tblAssets.Userdomain,
tblADusers.UPN,
tblAssets.IPAddress,
tblAssets.Description,
tblAssetCustom.Manufacturer,
tblAssetCustom.Model,
tblOperatingsystem.Caption As [Operating System],
tblAssets.SP As SP,
Case
When tsysOS.OSname = 'Win 7' And tblAssets.SP = 1 Then DateDiff(dd,
GetDate(), '01/14/2020')
End As DaysRemaining,
tblAssetCustom.Location,
tsysIPLocations.IPLocation,
tblAssets.Firstseen,
tblAssets.Lastseen,
tblAssets.Lasttried,
Case
When tsysOS.OSname = 'Win XP' Then '#ffadad'
When tsysOS.OSname = 'Win 7' And tblAssets.SP <> 1 Then '#ffdb8e'
When tsysOS.OSname = 'Win 7' And tblAssets.SP = 1 Then '#ffbe38'
When GetDate() > '01/14/2019' And GetDate() < '01/14/2020' And
tsysOS.OSname = 'Win 7' And tblAssets.SP = 1 Then '#ff8b00'
When GetDate() > '01/14/2020' And tsysOS.OSname = 'Win 7' And
tblAssets.SP = 1 Then '#ffadad'
Else '#d4f4be'
End As backgroundcolor
From tblAssets
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tsysOS On tsysOS.OScode = tblAssets.OScode
Left Join tsysIPLocations On tsysIPLocations.LocationID = tblAssets.LocationID
Inner Join tblComputersystem On tblAssets.AssetID = tblComputersystem.AssetID
Inner Join tblOperatingsystem On
tblAssets.AssetID = tblOperatingsystem.AssetID
Inner Join tblADusers On tblADusers.Username = tblAssets.Username And
tblADusers.Userdomain = tblAssets.Userdomain
Where tblComputersystem.Domainrole < 2 And tblAssetCustom.State = 1
Order By tblAssets.Domain,
tblAssets.AssetName