@Jay ,
Welcome to the Microsoft Q&A Platform. Thank you for reaching out & I hope you are doing well.
Unfortunately, I am unable to repo the issue.
- I just did a Lab (via Portal) and I was able to create a VNET Flow Log with Traffic Analytics enabled.
- Can you please try it again ?
- If you are using regular portal, try preview portal : https://portal.azure.com/?preview=yes
- If you are using preview portal, try normal portal : https://portal.azure.com/?preview=no
If the issue persists,
- Did you try with different user accounts?
- Did you try to use a different VNET/Subnet (for testing)?
- May I ask the region you are trying to create a VNET Flow Log
- Can you try to use a different region(for testing)?
Cheers,
Kapil