In your situation, how about the following sample script?
Sample script:
function myFunction() {
const ss = SpreadsheetApp.getActiveSpreadsheet();
const [sheet1, sheet2] = ["Sheet1", "Sheet2"].map(s => ss.getSheetByName(s));
const lastRowSheet1 = sheet1.getLastRow();
const sheet1Obj = sheet1.getRange("A2:B" + lastRowSheet1).getValues().reduce((o, [a]) => (o[a] = true, o), {});
const sheet2Values = sheet2.getRange("A1:A" + sheet2.getLastRow()).getValues();
const values = sheet2Values.filter(([a]) => !sheet1Obj[a]);
if (values.length == 0) return;
sheet1.getRange(lastRowSheet1 + 1, 1, values.length, 1).setValues(values);
}
When this script is run, the values of "Sheet1" and "Sheet2" are checked. When the values of column "A" of "Sheet2" are not included in the values of column "A" of "Sheet1", those values are appended to column "A" of "Sheet1". I thought that this is the goal you expect.
Note:
If you change the sheet name, please modify ["Sheet1", "Sheet2"].
References:
getValues()
setValues(values)
reduce()
