Need help with powershell to set sendon behalf on mailboxes from a csv

foreach ($user in (Import-Csv c:\rm\sendas.csv)) {
  Set-Mailbox $user.analyst -GrantSendOnBehalfTo @{add=$($user.manager)} -WhatIf
}

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this is what I get when I run
Cannot process argument transformation on parameter 'GrantSendOnBehalfTo'. Cannot convert value
"System.Collections.Hashtable" to type
"Microsoft.Exchange.Data.MultiValuedProperty`1[Microsoft.Exchange.Configuration.Tasks.RecipientIdParameter]". Error:
"MultiValuedProperty collections cannot contain null values.
Parameter name: item"
    + CategoryInfo          : InvalidData: (:) [Set-Mailbox], ParameterBindin...mationException
    + FullyQualifiedErrorId : ParameterArgumentTransformationError,Set-Mailbox
    + PSComputerName        :

How did you populate the Manager column of the .csv file?

attached csv

OK I figured out one issue but now when I run it gets to the empty columns under managers and errors out.  any ideas?

cannot process argument transformation on parameter 'GrantSendOnBehalfTo'. Cannot convert value
System.Collections.Hashtable" to type
Microsoft.Exchange.Data.MultiValuedProperty`1[Microsoft.Exchange.Configuration.Tasks.RecipientIdParameter]". Error:
Failed to convert  from System.String to Microsoft.Exchange.Configuration.Tasks.RecipientIdParameter. Error:
arameter values of type Microsoft.Exchange.Configuration.Tasks.RecipientIdParameter can't be empty. Specify a value
nd try again.
arameter name: identity"
   + CategoryInfo          : InvalidData: (:) [Set-Mailbox], ParameterBindin...mationException

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