Burping with a sour or bitter taste in the mouth but no food coming up usually points to acid reflux, where stomach acid travels partway up the food pipe. Since your tests confirm it's gastric and not something else, diet and routine changes are often the biggest lever to control it day to day.
Next Steps
Eat smaller, more frequent meals instead of large ones, avoid lying down for a couple of hours after eating, and cut back on spicy, oily, caffeinated and carbonated foods and drinks. Chewing slowly and not skipping meals also helps.
Health Tips
If the burping, taste or discomfort doesn't ease with these changes over a few weeks, please see a gastroenterologist for a proper evaluation.