Cannot change SharePoint permissions (400 - access denied)

[Beat]

I want to use a flow to grant a specific user Edit permissions upon creation or modification of a list item.
I'm the site collection administrator, the site owner, the list owner, and list permissions are unique (not inherited).

Here the flow:

Inputs:

Outputs:
{
"request": {
"url": null,
"traceId": "0HLPASQHTKR7S8924954c18"
},
"error": {
"code": "ApplicationResultException",
"message": "Access denied. You do not have permission to perform this action or access this resource.",
"date": "2019-08-27T12:30:54.4629855Z"
}
}

What could be the point I'm missing?

[Evgeniy]

Hello, @Beat

Could you try another type of API key? Please generate an "App auth" API key in the Plumsail account and then update the Plumsail connection in MS Flow that you use in the failed flow.