Hi,
Please carefully follow my instructions below to create a new Service and subscription limits (quotas) request quota increase to allow access to deploy SQL Database Managed Instance.
1. Navigate to this link to start the process:
https://portal.azure.com/#create/Microsoft.Support
2. Enter quota in the box and click Go

3. Select Others / Service and subscription limits (quotas) and click Next

4. Click Create a support request button, as shown

5. Select Issue type: Service and subscription limits (quotas), Subscription, Quota type: SQL Database Managed Instance, click Next

6. Now you should be on Additional details tab. Click Enter details

7. Select Region, enter new limits for Subnet and vCore, and then click Save and continue
8. Finish filling out Additional details tab, click Next, review and click Create. You should receive an automated email within a few minutes.
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP