All praise is due to Allah, and may peace and blessings be upon the Messenger of Allah.
First: Can Satan take the form of a human?
Yes. It is authentically established that Satan may sometimes appear in human form. However, there is no evidence that this happens regularly or continuously.
One piece of evidence is the hadith of Abu Hurairah (may Allah be pleased with him). The Prophet ﷺ appointed him to guard the Zakat collected during Ramadan. A devil then came to him in the form of a man and began taking handfuls of food. Abu Hurairah caught him three times. Each time, the devil claimed that he was in need and promised not to return.
On the third occasion, the devil taught him to recite Ayat al-Kursi before going to sleep. The Prophet ﷺ then said to him:
“Indeed, he told you the truth, although he is a liar. Do you know whom you have been speaking to for the past three nights, O Abu Hurairah?” He replied, “No.” The Prophet ﷺ said, “That was Satan.” Narrated by Al-Bukhari.
This hadith is authentic evidence that Satan can take the form of a human.
Second: Can humans always see the jinn and devils?
The basic principle is that jinn and devils are concealed from human sight. Allah, the Exalted, says:
“Indeed, he sees you, he and his tribe, from where you do not see them.” [Al-A'raf: 27]
Therefore, their appearing in forms that humans can see is an exceptional occurrence. It is not permissible to assert that it happens in every case without evidence or strong indications.
Conclusion: It is established in the authentic Sunnah that Satan can take the form of a human, as happened with Abu Hurairah (may Allah be pleased with him). However, it is not correct to believe that this is a regular or continuous occurrence, since the basic principle is that jinn are concealed from human sight.
And Allah knows best.