Adding buttons side by side will require that you’re in a space to do that. If you have multiple columns and there’s content in the way, then you will not be able to put the buttons side by side. Can you please provide a URL or more information on the page that you’re trying to add the buttons so that we can try to duplicate the issue and explain the issue with more clarity.
You can check out this image to see that I was able to add 2 buttons in the same row:
The option may be disabled due to content on the page or the format of the grids in the area where you’re trying to add them.