Question

is is trure that Non democratic countreis do not have constitution

Answer 1

Only two countries in the world do not officially call themselves democratic; they are Myanmar (Burma) which is a Military Dictatorship; and Vatican City, which is a Absolute Theocratic Monarchy. 
Many of the countries in the world that call themselves democracies are democratic in name only; and due to corruption, disenfranchisement and other factors are not true democracies.

following are steps by which government become  non democratic
The unsatisfied steps of a government make it non-democratic when there has been on consultation with the people and elected representatives. When a government does not discuss its country's affairs in assembly and when it makes decisions without taking into account the opinion of its parliament, it is also acting in an undemocratic manner. Members of a government are chosen by a nation through election. The government should fulfill the hopes of its nation.

Answer 2

Some Countries have ,Some countries don't
Non - Democratic Countries which neverthless have a Constitution. The Appropriate meaning of constitution is the countries which have correct set of rules and the government itself follow the rules.