When I saw your script, it seems that you are trying to run Google Apps Script at the Javascript side. I think that this is the reason for your issue. And also, when campnames[1]; is output, the 2nd element of the array campnames is used. I thought that this might be your 2nd issue.

If you want to retrieve the values from Spreadsheet on the Google Apps Script side and show the values in the dropdown list of the HTML side, how about the following modification?

HTML side:

From:

<div>
    <select>
      <option>
        <script>
          var sheet = SpreadsheetApp.openById("SPREADSHEETID").getSheetByName('[PAID] #1-Activity tracker');
          var myrange = sheet.getRange(7,6,1000);
          var campnames = myrange.getValues();
          campnames[1];
        </script>
      </option>
  </select>
</div>

To:

<div><select><?!= data ?></select></div>

Google Apps Script side:

From:

function mediaplanAdminSidebar() {
  var widget = HtmlService.createHtmlOutputFromFile('mediaplan').setTitle('HELP');
  SpreadsheetApp.getUi().showSidebar(widget)
  SpreadsheetApp.newTextStyle().setBold(true)
}

To:

function mediaplanAdminSidebar() {
  var sheet = SpreadsheetApp.openById("SPREADSHEETID").getSheetByName('[PAID] #1-Activity tracker');
  var myrange = sheet.getRange(7, 6, 1000);
  var campnames = myrange.getValues();
  var widget = HtmlService.createTemplateFromFile('mediaplan');
  widget.data = campnames.reduce((s, [e]) => s += `<option value="${e}">${e}<\/option>\n`, "");
  SpreadsheetApp.getUi().showSidebar(widget.evaluate().setTitle('HELP'));
}

Note:

• About the maximum number of options, I thought that this thread might be useful. Ref

Reference:

• HTML Service: Templated HTML