There are two ways to access locations.

Edit / Locations requires exclusive access.

Edit / Item Maintenance / Item List for Location does not require exclusive access and can be used when Order Entry is in use. If you are dealing with a single new item, use this one to assign it to locations.

If you are dealing with the addition of many items at once, it may be easier to wait for day end and use the items or item range functions in Edit / Locations.

If you normally add items to all locations, you can set the System Options in the Company Profile to enable "Allow items at all locations".