Cleanup math and LastUpdate fix

pallets/admin-utils/src/tests/mod.rs

@@ -2638,7 +2638,7 @@ fn test_trim_to_max_allowed_uids() {
                 NetUid::from(42),
                 new_max_n
             ),
-            pallet_subtensor::Error::<Test>::MechanismDoesNotExist
+            pallet_subtensor::Error::<Test>::SubnetNotExists
         );
 
         // New max n less than lower bound

pallets/subtensor/src/coinbase/root.rs

@@ -369,7 +369,7 @@ impl<T: Config> Pallet<T> {
         // 1. --- The network exists?
         ensure!(
             Self::if_subnet_exist(netuid) && netuid != NetUid::ROOT,
-            Error::<T>::MechanismDoesNotExist
+            Error::<T>::SubnetNotExists
         );
 
         // 2. --- Perform the cleanup before removing the network.