Tourist visiting Nepal must take visa from Nepal’s Diplomatic Mission in their
respective country.
In absence of Nepal's Diplomatic Mission or in case of difficulty in availing visa in
the home country, visitors arriving by air can get on arrival visa from the
Immigration desk at Tribhuvan International Airport.
However, such tourists must have a pre-visa recommendation letter issued by
Department of Tourism (DOT) OR Nepal Tourism Board (NTB).

DOT or NTB shall issue the visa recommendation letter; only after submission of
all required documents by the concerned local agency in Nepal.

https://trade.welcomenepal.com/wp-content/uploads/2021/04/protocol-final-1.pdf