It's currently only possible to create IP Locations using IP Ranges that don't overlap with existing IP Location ranges at all. Meaning that even if a single IP address in the range you're trying to add is also part of another IP location, you won't be able to add said IP location.
We recommend double-checking under Configuration\Asset Grouping that there is no overlap at all. You can also check your IP locations with the report below (add via Reports\Create New Report)
Select Top 1000000 tsysIPLocations.LocationID,
tsysIPLocations.StartIP,
tsysIPLocations.EndIP,
tsysIPLocations.IPLocation,
tsysIPLocations.Realstart,
tsysIPLocations.Realend
From tsysIPLocations