Given the code presented, it would be worth taking a step back and making this a teachable moment: don't just offer a correction, but explain why the original didn't work as expected.
Logical operators have an order of precedence like algebraic operators: exponents, parentheses, multiplication/division, addition/subtraction. x = 2 + 3 x 4 produces a different result than x = (2 + 3) x 4. In this case, it's parentheses, NOT, AND, OR.
If you take a look at your WHERE clause, you'll see that it's a bit of a mess and is at the root of your bad results.
Where
(tblAssets.AssetID Not In (Select tblUsers.AssetID From tblUsers
Where tblUsers.Name = 'LOCALADMIN')
And tsysOS.OSname = 'Win 2019')
Or (tsysOS.OSname = 'Win 2016')
Or (tsysOS.OSname = 'Win 2012 R2')
Or (tsysOS.OSname = 'Win 2012')
Or (tsysOS.OSname = 'Win 2008 R2')
Or (tsysOS.OSname = 'Win 2008' And tblAssetCustom.State = 1)
IF the OS = "Win 2019" and there's no localadmin account, include it (don't care whether the asset is active)
OR IF the OS = "Win 2016", include it (don't care about a localadmin account or whether the asset is active)
OR IF the OS = "Win 2012 R2", include it (don't care about a localadmin account or whether the asset is active)
OR IF the OS = "Win 2012", include it (don't care about a localadmin account or whether the asset is active)
OR IF the OS = "Win 2008 R2", include it (don't care about a localadmin account or whether the asset is active)
OR IF the OS = "Win 2008" AND the asset is active, include it (don't care about a localadmin account)
What I expect you actually meant was
Where
tblAssetCustom.State = 1
AND tblAssets.AssetID Not In (Select tblUsers.AssetID From tblUsers
Where tblUsers.Name = 'LOCALADMIN')
AND ( tsysOS.OSname = 'Win 2019'
OR tsysOS.OSname = 'Win 2016'
OR tsysOS.OSname = 'Win 2012 R2'
OR tsysOS.OSname = 'Win 2012'
OR tsysOS.OSname = 'Win 2008 R2'
OR tsysOS.OSname = 'Win 2008'
)
Because you're working with a fixed list of OSname values, you could also use
Where
tblAssetCustom.State = 1
AND tblAssets.AssetID Not In (Select tblUsers.AssetID From tblUsers
Where tblUsers.Name = 'LOCALADMIN')
AND tsysOS.OSname IN ('Win 2019', 'Win 2016', 'Win 2012 R2', 'Win 2012', 'Win 2008 R2', 'Win 2008')
IF the asset is active
AND there's no localadmin account
AND (the asset is running one of these OS versions)
Andy.S's proposal refines things with one assumption. If, for example, you have any machines running Win 2003 or 2003 R2, they would show up in the results. It's easy enough to add another condition to filter those out.