sharepoint online – My flow in Power Automate gives an error

I try to create a flow in Power Automate from an article I found on the internet:
John Liu article
But I get an error saying: bad format JSON-stream

What I try to do is: Change a value from the metadata column “Modified By” of a certain row in a Sharepoint list with PowerAutomate.

But it seems the JSON-code isn’t correct:

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little question about the uri:

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Does “items(1)” refer to the first row of the sharepoint list?

Greetings, Peter