This is completely normal. There are thousands of nodes and wallets all over the globe constantly trying to stake the next block on the Neblio blockchain and sometimes there are conflicts.
This happens when your node or wallet successfully stakes the next block, but someone else did too and their block was accepted by the network before yours was. This can happen for a variety of reasons. This is not an error unless it is happening very frequently. Also, when this happens the token age does not reset, so the next time you successfully stake the stake reward will be larger as your tokens have continued to gain age.
There are ways to lower the chances of this happening or identify why this is happening if it does so frequently.
1) It seems to happen more on mac than other platforms. This is due to some timing in macOS that Neblio cannot control, the Neblio wallet code is the same for mac/linux/windows. Some users have seen this happen much less after switching their staking to something like a Raspberry Pi.
2) Better network performance can lower the chance of this happening. The quicker your block is sent out to the network, the more likely you are to have the block accepted. Switching from wifi to ethernet, or making sure you have a good internet connection are good tips.
3) Very low connection count can increase the chances of this happening. In general you should have at least 8 connections to the network.