Rubén is correct you are entering values that don't match a valid number format so Sheets is interpreting them as text.
When you then perform your division calculation on the text Sheets tries again to interpret them as numbers but fails.
This step is not necessary if you modify the durations manually. If it isn't practical to edit all the durations or you simply prefer to continue entering times the way you have been, the following modified formula will resolve the issue:
To calculate a range of values with a single formula you could use an ARRAYFORMULA:
("0:" &C3:C20)/1.3, ""))
Apply the following custom number format to the cell(s) with your formula for durations with milliseconds:
You can tweak the custom number format. In the table below, the only thing that is changed in each row is the custom number format.
- The first column contains a text value representing a duration.
- The first row shows the underlying number to which all the custom formats are applied.
||Pace (Duration ÷ 1.3)
||Duration (Pace × 1.3)