Are all the variables printing negative or just the UVI value? That info will be helpful to try and troubleshoot but, regardless, I think it may be related to this issue
where, if the light source is intense enough, the UV values will print negative. Try adjusting the distance and the angle you are pointing the light source at the sensor and see if that helps.
You could also try configuring your VEML6075 with Example 2
from the library to help but those adjustments are more for the performance of the sensor's output, not so much accounting for light level intensity. If your project using this sensor is just going to be recording UVA and UVB data from ambient light (eg. sunlight or another light source), you should not have to worry about negative values but if you are trying to record irradiance from a strong UV light source, we can try and figure out a way to fix it or filter out the bad data.