Yes letters and other mail can get lost in the mail through the USPS system.
The USPS delivers millions of letters and packages throughout the month and year and no matter how good and efficient and careful the USPS is they can still make mistakes and cause a letter or something else to get lost in the mail.
I've had rent payments before get lost in the mail and then show up a month later to the landlord.
I've also had other bills get lost in the mail when I sent them and I've also gotten other peoples mail and letters in my own mail box.
Most mail is delivered correctly though to the correct addresses on time but sometimes things can happen and cause the letter or package or other mail to get lost in the system.
There are humans working at the USPS and we all make mistakes.