You will need a tourist visa (http://www.nepalimmigration.gov.np/page/tourist-visa) to participate in a conference in Nepal.
The “On Arrival” visa procedure is very quick and simple, except for nationals of designated countries.
Nationals of designated countries (http://www.nepalimmigration.gov.np/post/prohibition-to-visa-on-arrival-1) are requested to acquire a visa prior to their arrival from nearby Nepali diplomatic missions, i.e. embassies or consulates. If you need a letter of invitation for visa, please contact us at firstname.lastname@example.org.
Visa policy of Nepal
The visa policy of Nepal is relatively liberal, allowing citizens of almost all nations to obtain a tourist visa on arrival. In accordance with the law, citizens of all countries except India require a visa to enter Nepal.
All tourists are allowed to stay in Nepal for a maximum of 150 days in one calendar year. Visitors must hold passports that are valid for at least 6 months from the date of arrival.
Visa on arrival
With the exception of nationals of states mentioned below and holders of refugee travel documents, any foreign national can obtain a tourist visa on arrival in Tribhuvan International Airport, the only airport serving Kathmandu, the capital of Nepal. Multiple entry visas can be issued for a duration of stay of 15, 30 or 90 days. Holders of temporary passports are not eligible unless they hold a temporary passport issued by a European Union member state.
Visa required in advance
Residents of the following countries are required to apply for a visa prior to arrival in Nepal:
Afghanistan, Cameroon, Eswatini, Ethiopia, Ghana, Iraq, Liberia, Nigeria, Palestine, Somalia, Syria, and Zimbabwe
Visa fee waiver
Citizens of South Asian Association for Regional Cooperation (SAARC) member countries can receive a tourist visa for 30 days free of charge and the countries include:
Afghanistan, Bangladesh, Bhutan, Maldives, Pakistan, and Sri Lanka.
Additional visa information can be found at http://www.nepalimmigration.gov.np/page/visa.