Google Sheets App Script If Else not working

You are logging News not found for each comparisons with all the rows of Test2, which is improper.

Instead, you should only log when the comparison is finished.

function test() {
  var sheet = SpreadsheetApp.getActiveSpreadsheet();
  var source_sheet = sheet.getSheetByName("Test1");
  var target_sheet = sheet.getSheetByName("Test2");
  var lastRow = source_sheet.getLastRow();
  var inputs = source_sheet.getRange('A' + lastRow).getValues().flat();
  var days = target_sheet.getRange('A1:A').getValues().flat();
  var codes = target_sheet.getRange('B1:B').getValues().flat();

  inputs.forEach(function(input, count){
    let found = false;
    for(var i = 0; i < days.length; i++){
      if(days[i].trim() == input.trim()){
        source_sheet.getRange('B' + (count + source_sheet.getLastRow())).setValue(codes[i]);
        Logger.log(codes[i]);
        found = true;
        break;
      }
    }
    if (!found) { Logger.log("News not found"); }
  });
}

Or simply,

function test() {
  var sheet = SpreadsheetApp.getActiveSpreadsheet();
  var source_sheet = sheet.getSheetByName("Test1");
  var target_sheet = sheet.getSheetByName("Test2");
  var lastRow = source_sheet.getLastRow();
  var inputs = source_sheet.getRange('A' + lastRow).getValues().flat();
  var days = target_sheet.getRange('A1:A').getValues().flat();
  var codes = target_sheet.getRange('B1:B').getValues().flat();

  inputs.forEach(function(input, count){
    for(var i = 0; i < days.length; i++){
      if(days[i].trim() == input.trim()){
        source_sheet.getRange('B' + (count + source_sheet.getLastRow())).setValue(codes[i]);
        Logger.log(codes[i]);
        return;
      }
    }
    Logger.log("News not found");
  });
}

function test() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sh1 = ss.getSheetByName("Sheet0");
  var sh2 = ss.getSheetByName("Sheet1");
  var lastRow = sh1.getLastRow();
  var vs1 = sh1.getRange(1,1,sh1.getLastRow()).getValues().flat().map(e => e.trim());
  var vs2 = sh2.getRange(1,1,sh2.getLastRow(),2).getValues();
  vs2.forEach(r => {
    if(~vs1.indexOf(r[0].trim())) {
       Logger.log(r[1]);
    }
  });
}