This could be down to a number of factors.
- The desired size may genuinely not be available in the region. Not all regions have every VM SKU.
- You may not have quota for that VM SKU for the subscription you're operating in. see: https://learn.microsoft.com/en-us/azure/quotas/quickstart-increase-quota-portal
- There could be a capacity issue in the region and your are not currently able to leverage that VM SKU
I'd suggest trying some other VM SKU's and seeing if it succeeds, this could narrow down the particular problem.