What are all the countries that have a value added tax? I'm a web developer (not an economist) and i'm working on a program that deals with value-added tax (VAT), maybe it is a stupid question but, is the value added tax obligatory? I just want to know if this tax exists in every country all over the world?
No, many countries do not. For example. the United States does not have a VAT. According to the paper The Consequences of the Value-Added Tax on Inequality in Developing Countries (Alavuotunki and Pirttilä (2015)) here is an extensive list:

A4 – Countries and introduction year of VAT. (Source: IMF)  
Asia-Pacific    
1977    Korea, Republic of
1985    Indonesia
1986    New Zealand
1986    Taiwan Prov.of China
1988    Philippines
1989    Japan
1990    Pakistan
1991    Bangladesh
1992    Thailand
1994    China,P.R.: Mainland
1994    Singapore
1997    Nepal
1998    Mongolia
1998    Sri Lanka
1999    Cambodia
1999    Papua New Guinea
1999    Vietnam
2000    Australia
2005    India
no VAT  Brunei Darussalam
no VAT  China,P.R.:Hong Kong
no VAT  Lao People's Dem.Rep
no VAT  Malaysia
no VAT  Myanmar

Small Islands   
1973    San Marino
1990    Iceland
1992    Cyprus
1992    Fiji
1994    Samoa
1995    Malta
1997    Barbados
1998    Vanuatu
2004    Cape Verde
2007    Antigua and Barbuda
2007    St. Vincent &amp; Grens.
2010    St. Kitts and Nevis
2012    St. Lucia
2013    Seychelles
no VAT  Bahamas, The
no VAT  Comoros
no VAT  Grenada
no VAT  Maldives
no VAT  Solomon Islands
no VAT  São Tomé &amp; Príncipe
no VAT  Timor-Leste
no VAT  Tonga
no VAT  Tuvalu

Americas    
1967    Brazil
1968    Uruguay
1970    Ecuador
1973    Bolivia
1973    Peru
1975    Argentina
1975    Chile
1975    Colombia
1975    Costa Rica
1975    Nicaragua
1976    Honduras
1977    Panama
1980    Mexico
1982    Haiti
1983    Dominican Republic
1983    Guatemala
1990    Trinidad and Tobago
1991    Canada
1991    Jamaica
1992    El Salvador
1993    Paraguay
1993    Venezuela, Rep. Bol.
2006    Belize
2007    Guyana
no VAT  United States

North Africa and Middle East    
1976    Israel
1985    Turkey
1986    Morocco
1988    Tunisia
1991    Egypt
1992    Algeria
2001    Jordan
2002    Lebanon
2008    Iran, I.R. of
2009    Djibouti
no VAT  Afghanistan, I.R. of
no VAT  Bahrain, Kingdom of
no VAT  Iraq
no VAT  Kuwait
no VAT  Libya
no VAT  Oman
no VAT  Saudi Arabia
no VAT  Syrian Arab Republic
no VAT  United Arab Emirates
no VAT  Yemen, Republic of

Sub-Saharan Africa  
1960    Côte d'Ivoire
1980    Senegal
1986    Niger
1989    Malawi
1990    Kenya
1991    Benin
1991    Mali
1991    South Africa
1993    Burkina Faso
1994    Madagascar
1994    Nigeria
1995    Gabon
1995    Mauritania
1995    Togo
1995    Zambia
1996    Guinea
1996    Uganda
1997    Congo, Republic of
1998    Ghana
1998    Mauritius
1998    Tanzania
1999    Cameroon
1999    Mozambique
2000    Chad
2000    Namibia
2001    Central African Rep.
2001    Rwanda
2002    Botswana
2003    Ethiopia
2003    Lesotho
2004    Zimbabwe
2009    Burundi
2010    Eritrea
2013    Swaziland
no VAT  Angola
no VAT  Gambia, The
no VAT  Guinea-Bissau
no VAT  Liberia
no VAT  Sierra Leone
no VAT  South Sudan

Europe  
1967    Denmark
1968    France
1968    Germany
1969    Netherlands
1969    Sweden
1970    Luxembourg
1970    Norway
1971    Belgium
1972    Ireland
1973    Austria
1973    Italy
1973    United Kingdom
1986    Portugal
1986    Spain
1987    Greece
1994    Finland
1995    Switzerland

Central Europe and the ex-Soviet    
1988    Hungary
1992    Armenia
1992    Belarus
1992    Estonia
1992    Georgia
1992    Kazakhstan
1992    Kyrgyz Republic
1992    Latvia
1992    Lithuania
1992    Moldova
1992    Russian Federation
1992    Tajikistan
1992    Turkmenistan
1992    Ukraine
1992    Uzbekistan
1993    Czech Republic
1993    Poland
1993    Romania
1993    Slovak Republic
1994    Bulgaria
1996    Albania
1998    Croatia
1999    Slovenia
2000    Macedonia, FYR
2003    Montenegro
2003    Serbia, Republic of
2006    Bosnia &amp; Herzegovina
no VAT  Kosovo, Republic of