If your licensed users for Microsoft Purview Encryption don't have the encryption button showing up in Outlook desktop or web, you may have to enable Azure/Entra RMS using the commands below and it should show up shortly after (wait 15-30 minutes, maybe more). Also, you may need to update MS Office in File > Account > Update Options > Update Now.
In elevated Powershell:
Set-ExecutionPolicy RemoteSigned
Connect-ExchangeOnline
Install-Module AIPService
Connect-AipService
Enable-AipService
$RMSConfig = Get-AadrmConfiguration
$LicenseUri = $RMSConfig.LicensingIntranetDistributionPointUrl
Set-IRMConfiguration -LicensingLocation $LicenseUri
Set-IRMConfiguration -AzureRMSLicensingEnabled $true
Set-IRMConfiguration -InternalLicensingEnabled $true
Set-IRMConfiguration -SimplifiedClientAccessEnabled $true
Get-IRMConfiguration