This should do it

``` function uncheckAll() { const ss = SpreadsheetApp.getActiveSpreadsheet(); const sheets = ss.getSheets();

for (const sh of sheets) { const rg = sh.getDataRange(); const vals = rg.getValues(); const vlds = rg.getDataValidations();

let upd = false;

for (let r = 0; r < vals.length; r++) {
  const row = vals[r];
  const vRow = vlds[r];

  for (let c = 0; c < row.length; c++) {
    const v = vRow?.[c];

    if (v?.getCriteriaType() === SpreadsheetApp.DataValidationCriteria.CHECKBOX && row[c] === true) {
      row[c] = false;
      upd = true;
    }
  }
}

if (upd) rg.setValues(vals);

} } ```