Is it necessary to repeat the prayer if there are traces of impurity on the underwear due to the difficulty of completely removing them because of scarcity of water, knowing that the available clothes are very few?

The Companions were unanimous on the permissibility of suffice with stones to remove impurity after answering the call of nature, and tissues take the place of stones. The legislation of permissibility of using stones for purification (istijmar) means that complete purification is not mandated, for stones or tissues remove the substance of the impurity, and what remains of its traces is pardoned. It is best to use stones or tissues first, then water if available. As for impurity on clothes, it must be removed by whatever means possible. If that is difficult and one fears the time for prayer will expire, then pray with your clothes as they are, and there is no blame.