Must a new professional inform a client or patient that he is new to his profession, given that he was not asked about his experience, assuming he would answer truthfully if asked?

You are not required to inform the customer that you are new to the job as long as you are proficient in your tasks. However, if you are not proficient, it is not permissible for you to undertake treatment and deal with people's bodies and money without knowledge. Rather, you must apologize to the customer. Practicing medicine without knowledge is considered an transgression, and the physician guarantees what results from errors thereof, due to the saying of the Prophet, peace and blessings be upon him: "Whoever practices medicine, and no prior knowledge of medicine was known from him, then he is liable." And his saying: "Any physician who practices medicine on a people, and no prior medical practice was known from him, and he causes harm, then he is liable." So, whoever treats a patient and transgresses in his treatment, he is liable for what occurs.