Got the solution
when I tried to click on the button a second time alert Keywords were able to identify the alert.
so I implemented the following to handle it
by referring Alerts are displayed, but disappear before I can accept it - #29 by Matthew_Loo
WebUI.click(findTestObject('Web/Sales Order Upload/Upload Data button'))
try { WebUI.delay(2)
WebUI.verifyAlertPresent(1)
}
catch(Exception ex) {
WebUI.switchToDefaultContent()
WebUI.click(findTestObject('Web/Sales Order Upload/Upload Data button'))
}