I can use the VSTS.Contents to get the capacities, please try to use the following Query:
let
Source = VSTS.Contents("https://dev.azure.com/{org}/{project}/{team}/_apis/work/teamsettings/iterations/{iterationId}/capacities/{userId}?api-version=6.0"),
#"Converted to Table" = Table.FromRecords({Json.Document(Source)})
in
#"Converted to Table"
I can use the VSTS.Contents to get the capacities, please try to use the following Query: