Just got this error when configuring an ASA, took me a bit to figure out what the issue was.
When trying to create a static NAT rule with port 443 you get the error:
unable to reserve port 443 for static PAT
The issue was that under Management access, ASDM access was granted on the Outside interface. Since ASDM runs over HTTPS that was the issue. As soon as I removed that rule I was able to create my NAT without issues.