The command you chose cannot be performed with multiple selections.
Select a single range and click the command again.
The error happens if you have multiple cells selected that aren’t next to each other. If you only have one cell selected, it may be a merged cell or an error. In the later case, where it’s incorrectly reporting as multiple selections, edit the contents of the cell and try again. Or edit the contents of the cell and copy the tect within the cell, rather than the cell itself.