I needed to make a small change. There was a missing closing bracket, and the way the OU column works requires a Like operator, since it displays the FQDN, not just a single OU. Thanks!
Select Top 1000000 tsysOS.Image As icon,
tblAssets.AssetID,
tblAssets.AssetName,
tblAssets.Domain,
tblAssets.Username,
tblAssets.Userdomain,
tblAssets.IPAddress,
tblAssets.Firstseen,
tblAssets.Lastseen,
tblAssets.Lasttried,
tblADComputers.OU
From tblAssets
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tsysOS On tsysOS.OScode = tblAssets.OScode
Inner Join tblADComputers On tblAssets.AssetID = tblADComputers.AssetID
Where tblAssets.AssetID Not In (Select Top 1000000 tblSoftware.AssetID
From tblSoftware Inner Join tblSoftwareUni On tblSoftwareUni.SoftID =
tblSoftware.softID
Where tblSoftwareUni.softwareName Like 'Skype for Business Basic 2016%') And
tblADComputers.OU Like '%Computers%'
Order By tblAssets.Domain,
tblAssets.AssetName