As taken from the official documentation: https://learn.microsoft.com/en-us/powershell/module/msonline/get-msoluser?view=azureadps-1.0
LicenseReconciliationNeeded. Whether or not the user currently has a mailbox without a license. In this case, the user should be licensed with 30 days to avoid losing their mailbox.
You fix it by assigning a license that includes Exchange Online plan, or by converting the mailbox to shared one (as long as its size is lower than 50GB and no archive is provisioned).