Numbers help with adding cells with 0

It appears your sum is of cells that have duration values in them except for the cell that has a 0, which is not a duration. Format that one cell or, better yet, format all cells that will be summed, as duration. When you enter a 0 or any other number into one of those cells, it will get formatted as a duration, not as a regular number, and your SUM will work. Either that or have all those cells be automatic and enter all your "durations" as regular numbers without any duration units. Then the SUM will be of all numbers and that should work also. There isn't enough of your table showing in the screenshot to say which of these is the best way to go.

Note that there is a special notation system for entering Duration values (instead of numbers).

For example if your language is set to English, for 1 day you would type in 1d, one hour is 1h, one minute is 1m, etc. If you enter a Duration value that Numbers recognizes, Numbers will right-align it by default.

SG