For starters, you're using "mb" instead of "gb", if this is for Exchange Online the size of the mailbox will be in GB. If the mailbox size is 100GB and you want the warning to be sent at 50% usage, use this:
Set-Mailbox -Identity "user name" -IssueWarningQuota 50GB -ProhibitSendQuota 80GB -ProhibitSendReceiveQuota 98GB -UseDatabaseQuotaDefaults $false