I think this will be because that configuration is not actually valid.
First, the pool start and end need to be network addresses rather than a start and end IP.
Second, you actually need at least 2 pools available as the system will consume the first pool for the wg0 interface.
I suggest you do a quick restart before making this change as I've seen a config like that can get the system into a muddle.
Then edit this setting and put something like 172.16.1.0 for the start and 172.16.2.0 for the end, with a pool prefix of 24.
This should reserve the range 172.16.2.1-172.16.2.254 for your clients.