Sorry for being a bit late, but for anyone out there with the same issue, you cannot edit this value in the GUI, but you can set this value (and any other columns) with PowerShell like this:
Connect-PnPOnline $siteCollURL -CurrentCredentials
$documentRelativeURL = "/Publicering%20Metadata/Sp%C3%B6ket.txt"
$file = Get-PnPFile -Url $documentRelativeURL -AsListItem
Set-PnPListItem -List "Publicering%20Metadata" -Identity $file.Id -Values @{"ComplianceAssetId" = "somevalue"}
You can set a default value for the library like this:
Set-PnPDefaultColumnValues -List "Publicering%20Metadata" -Field “ComplianceAssetId” -Value “123”
