Difficulty peeing on MDMA is commonly reported and usually temporary. Likely reasons include sympathetic nervous system activation that keeps the urethral sphincter tight, a possible rise in vasopressin that reduces urine production, pelvic muscle tension, and being tense or inward-focused, which can make it harder to let go.
What to try: take your time and do not force it. Sit down, relax your belly and pelvic floor, breathe slowly, and let a tap run or listen to water. Sip fluids instead of chugging. If nothing happens, wait 15 to 30 minutes and try again.
Be cautious if you have drunk a lot and still cannot pass any urine, if you feel painful pressure low in the abdomen, or if the issue persists well after the effects fade. In those cases, consider seeking medical help. For a concise overview with tips, see this Q&A: why peeing can be hard after MDMA.
Stay safe.