how to use if condition in .map function?

You're missing a return in your .map. Notice that inside the if statement I am returning the items being mapped.

renderall() {           
  return this.state.leader.map(alb =>  {
    if(alb.Renk == 'MAVI') {
      return (    
        <View style={styles.container} key={counter = counter + 1}> 
          <Text style={[styles.textStyle, {marginLeft:'5%'}]}> {alb.Tescil_No}  </Text>

          <Text style={[styles.textStyle, {marginLeft:'6%'}]}> {alb.GumrukAdi}  </Text>

          <Text style={[styles.textStyle, { marginLeft:'5%'}]}> {alb.ACIKLAMA}   </Text>                                                
          </View>
      );
    }
  });
}

This article explains in more detail how the .map function works https://codeburst.io/learn-understand-javascripts-map-function-ffc059264783

Here is a very small example showing how to use the .map function. Notice the return statement.

let leaders = ['Tom','Jerry','Mike'];

let mappedLeaders = leaders.map((leader, index) => {
  return `${leader} is number ${index}`; // notice that I am returning here
})

console.log(mappedLeaders)

Here is an example of using a .map with an if statement inside it. Notice we will get undefined for the first item in the mappedLeaders as we are not returning anything for the first item because Tom is excluded due to the fact that his name is too short.

let leaders = ['Tom','Jerry','Mike'];

let mappedLeaders = leaders.map((leader, index) => {
  if (leader.length > 3) {
    return `${leader} is number ${index}`; // notice that I am returning here
  }
});

console.log(mappedLeaders)