Hi @JoyK ,
According to your description, you want to get the last modified datetime in SharePoint list .
Here are the steps you can refer to :
(1)We can use the "Get items" to get the items in SharePoint List:
And this is the sample output:

(2)Then we can add a Select action to get all the "Modified" value:

(3)Then we can add a Compose action to get the max "Modified" value:
last(sort(body('Select'),'Modified'))['Modified']

Then we can get the max of the "Modified" in SharePoint List:

Best Regards,
Wearsky