I have MS Project Server 2013 and created Project Web App site
I have created multiple Administrative Time items now I want to
delete Administrative Time items but there is no option.
You can only delete categories that are not yet saved. For example, let's say that you have created several new categories on the Administrative Time page, but have not yet clicked Save on that page. You can delete any of the new categories that you created while on that page. Once you click Save on the Administrative Time page, the categories that you added can no longer be deleted.
Same information in Project Server 2013 Administrator's Guide.