Prayers of Zuhar and Ashr, and Maghrib and Isha can be combined only when there is a reason for it like a journey or when it is difficult for any reason to say them separately. Without any reason, it is not allowed.
Alla says:
فَإِذَا اطْمَأْنَنتُمْ فَأَقِيمُوا الصَّلَاةَ ۚ إِنَّ الصَّلَاةَ كَانَتْ عَلَى الْمُؤْمِنِينَ كِتَابًا مَّوْقُوتًا
But when you are in peaceful circumstances, offer the full prayer, [and, for this, pray at the appointed times] because the prayer is incumbent on the believers at the appointed times