I know this is an old query, but thought I'd explain a solution (and I will add more details to my blog soon)
When you use Get Items in Power Automate and you want to use the filter property, there's a couple of things to note. The Odata query is expecting the ID for the user from the User Information List, Modified By and Created By internal names are Editor and Author.
This is what you can do...
Do a get items to the Site Collection that hosts the library you want to get stuff out of, use a custom value in the List Name... type "User Information List" - without the quotes
On this step add the following filter where Display Name is the display name of the users you want to get
Title eq 'Display Name'
Just for debugging you can then make a compose function and set it to this expression (assuming the step name was "Get items") - this also assumes you don't have duplicate display names in your AD
first(outputs('Get_items')['body/value'])?['ID']
Then do a second Get Items on the list you want to filter Created By. In the filter section put in
Author eq 'Outputs'
(where outputs is the dynamic content output from the Compose step)
Hope it helps