X_BRANCH_EXISTS
Means
that branch database already exists
Typical cause
two conditions, one code: x db branch create <name> for a name already taken — refused rather than overwritten, because a silent re-clone discards whatever the branch already held — or a dropBranch() aimed at the database this session is itself connected to, which Postgres cannot drop from underneath its own connection
Fix
x db branch delete <name> # then re-create, or pick another name — x db branch list --json lists what exists. To drop the database you are connected to, reconnect elsewhere first: DATABASE_URL=.../postgres