X_MCP_NOT_BRANCH_DB
Means
db.migrate was aimed at a database that is not a branch
Typical cause
a production-tagged target, or any shared/long-lived database
Fix
x db branch create <name> # then retry db.migrate — the guidance rides behind # so the whole line pastes into a shell. A production target gets the other half of the refusal instead: run migrations in production through the migrate role: ROLE=migrate in your deploy hook