Sometimes converting a shared mailbox to an user mailbox is not working from the Exchange Admin console in Office 365.
If this is the case then you can try to convert the user mailbox with Powershell.
Note: Current Outlook connections to the shared mailboxes will stop working after you convert.
First set a Office license to the user. The user must have a valid MS Office license it is possible to have a Office Exchange mailbox.
(in the case you need additional support please contact 2tCloud Support)
Start PowerShell in Administrative mode
run the next commands:
Install-Module -Name AzureAD
$UserCredential = Get-Credential
Create a persistent connection to a local or remote computer (https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.core/new-pssession?view=powershell-7.1 )
Set-Mailbox [MAILBOXNAME] -Type Regular
Check Exchange Admin portal to see the mailbox is away in 'Shared Mailbox' , and is visible in 'Mailboxes' from the users
Optional: Maybe you want/need to add the mailbox again in the customers Outlook profile.