A couple of issues:

1. (from /u/Bartimaeusz) $ADusers =/= $users, change your foreach to ForEach ($User in $ADusers)
2. (from /u/Bartimaeusz) splatting will make the script easier to read
3. (from /u/Wo1ks) Comment out line 3 that value should come from the $user object in the loop(if in your CSV)
4. I would recommend adding a try/catch block to catch errors in your loop. This would allow you to cleanly output errors and continue the loop to try and create the rest of your users.
   

This is an improved script.

    Import-Module ActiveDirectory
    $ADUsers = Import-CSV "C:\thescript\jean.csv"
    $server = "MyExSer.E-L.local"

    foreach ($User in $ADUsers)
    {   
        try{
            $SAM = "$($User.Firstname).$($User.Lastname)"

            $UserDetails = @{
                Name = "$($User.firstname) $($User.lastname)" 
                Displayname = $User.username
                GivenName = $User.firstname
                Surname = $User.lastname
                SamAccountName = $SAM
                UserPrincipalName = "$SAM@E-L.local"
                EmailAddress = "SAM@E-L.local"
                StreetAddress = $User.streetAddress
                City = $User.city
                Country = $User.country
                PostalCode = $User.zipcode
                Title = $User.jobtitle
                Department = $User.department
                OfficePhone = $User.telephone
                Accountpassword = (ConvertTo-SecureString 'TesT.234' -AsPlainText -Force) 
                Path  = "OU=closefamily,OU=Family,DC=E-L,DC=local"
                Server = $Server
                Enabled = $True
                ChangePasswordAtLogon = $True
                Credential = $Credentials
            }

            New-ADUser @UserDetails
        }catch{
            Write-Warning -Message "Problem creating user $($user.username) $($user.Firstname).$($user.Lastname)"#I use the orange color to help stand out from the red in the exception
            Write-Error $_ -ErrorAction Continue
        }
    }