It is not available for free tier. Below are the listed supported subscription types:
SQL Managed Instance currently supports deployment only on the following types of subscriptions:
Enterprise Agreement (EA)
Pay-as-you-go
Cloud Service Provider (CSP)
Enterprise Dev/Test
Pay-As-You-Go Dev/Test
Subscriptions with monthly Azure credit for Visual Studio subscribers
Below is the Azure link for reference:
https://learn.microsoft.com/en-us/azure/azure-sql/managed-instance/resource-limits#supported-regions
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Regards
Kalyan