If this happens, the reason is that your asset isn’t set up to track its useage or service intervals according to the total number of miles / kilometers / hours.
This is set according to the ‘model’ of that asset. For example, if you set a “Citroen Relay” van useage ‘log-type’ to ‘not applicable’, rather than ‘total miles/km’, the system will not allow you to edit that model’s service-interval according to the ‘miles/km/hrs’.
To amend this setting, you need to go to ‘Categories / Models’. You should be able to find the relevant model in this list, and the ‘log type’ will also be displayed. Click the ‘Edit’ button, and change the log type.