It is a warning related to stacks mining and not relevant to sBTC transactions.
Wait until the stacks block height (stacks_tip_height) is 2 or more by checking http://localhost:3999/v2/info.
Initially, the server won't response.
For more detailed information about the progress you can look at ./logs.sh stacks-api.
Wait until you see a message like Proxy created: / -> http://stacks:20443
or
you can look at ./logs.sh stacks.
There is a time after Clarity state genesis was computed without log messages. Just keep on waiting...
See the bitcoin blocks syncing and wait for messages like Miner: mined anchored block ....