Essentially this means that the sun or moon will not be found at the given altitude or azimuth in the location and period of time you're searching. That might be for a number of reasons, discussed here.
The sun more or less repeats its motion each year, with only slight variations. If you're getting no results for the sun, and the search period is at least 1 year, then this will typically be because:
- The azimuth is outside the range at which the sun ever travels for the given location. For example, the sun rises at at azimuth of 57.9° on the summer solstice here in Boulder, Colorado. That means if you search for any azimuth farther north than that, you'll never get any results
- The altitude is higher than the maximum altitude the sun ever achieves for the location. On the summer solstice the sun culminates at +73.4° in Boulder, CO. If you search for an altitude higher than that, you'll get no results.
- The sun is never found at the specific azimuth and altitude, even if the values are in range. Typically you can increase your search tolerance to start finding results. However, using the Boulder, CO example, bear in mind that you won't ever find the sun at az:57.9° alt: +30.0° - the sun rises at that azimuth on the summer solstice, and so will be that far north and that high in the sky.
All the above considerations apply to the moon also, but with some added variations. Unlike the sun, the motion of the moon varies over an 18.6 year cycle, making things even less readily predictable to most people (hence apps!).
The extreme azimuths of moonrise and moonset are achieved only at the Lunar Standstill (every 18.6 years, the next one occurring in April 2025).
In addition, if you restrict your searches to only specific moon phases (e.g. full moon), or to specific phases of twilight, then you'll find even fewer results.
If you're seeing no results, the best approach is to remove restrictions (such as moon phase and twilight), and to increase the tolerance until you start seeing results.