Why doesn't the last if part work in onedit?

Only the first if part is executed repeatedly and it does not go to the next if part. I made a few dropboxes on different sheets. If you select one from the dropbox, another dropbox that matches the value is imported and pasted. how should i solve it

var sheet = SpreadsheetApp.getActiveSheet();

var spreadsheet = SpreadsheetApp.getActive();

function onEdit(e){

    if (sheet.getRange('C15').getValue() == 'line'){
    //Omit details
    spreadsheet.getRange('dropboxs!B2:C3').copyTo(spreadsheet. getRange('C18'),SpreadsheetApp.CopyPasteType.PASTE_NORMAL, false);
    }

    if (sheet.getRange('C18').getValue() == 'c'){
    //Omit details
    spreadsheet.getRange('dropboxs!B2:C3').copyTo(spreadsheet. getRange('C21'),SpreadsheetApp.CopyPasteType.PASTE_NORMAL, false);
    }
}

try to treat it independently:

var spreadsheet = SpreadsheetApp.getActive();

// This function is triggered whenever an edit is made to the spreadsheet
function onEdit(e) {
var sheet = e.source.getActiveSheet();
  
// Check if the value in cell C15 is 'line'
if (sheet.getRange('C15').getValue() == 'line') {
    
// Copy the range B2:C3 from the 'dropboxs' sheet to cell C18
spreadsheet.getRange('dropboxs!B2:C3').copyTo(spreadsheet.getRange('C18'), SpreadsheetApp.CopyPasteType.PASTE_NORMAL, false);
    
// Set a timed trigger to check the condition in cell C18 after the copy operation
ScriptApp.newTrigger('checkC18Condition')
.timeBased()
.after(500)  // Wait for 0.5 seconds before running the check
.create();
}}

// This function is triggered by the timed trigger created in onEdit
function checkC18Condition() {

// Ensure we're working with the active spreadsheet and sheet
  var sheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();
  
// Check if the value in cell C18 is 'c'
if (sheet.getRange('C18').getValue() == 'c') {

// Copy the range B2:C3 from the 'dropboxs' sheet to cell C21
spreadsheet.getRange('dropboxs!B2:C3').copyTo(spreadsheet.getRange('C21'), SpreadsheetApp.CopyPasteType.PASTE_NORMAL, false);
}}