What is the ruling on the prayer of one who performed tayammum and prayed, while he is a guard unable to leave his post to perform ablution and water is not available to him?

If you cannot find water or fear harm to your property or yourself because of it, then tayammum (dry ablution) is obligatory and permissible, and your prayer is valid, and you are not required to repeat it. However, if you are able to seek water without harm, then tayammum is not permissible, and you must repeat the prayer.