Hello Arun,
Thanks for reaching out to Microsoft Q&A platform.
Below articles should help you out: https://learn.microsoft.com/en-us/troubleshoot/azure/virtual-machines/allocation-failure
Sometimes retrying the same deployment should help out !
If you need additional help , please provide the Subscription ID , exact constraints error code displayed in the error message. kindly accept , if it helps
Thanks,
Deepanshu