X_DB_BRANCH_FAILED
Means
x db branch refused, or failed for a reason no framework error claimed
Typical cause
two shapes under one code. A deliberate refusal: delete aimed at a name x db branch list does not show, whose cause names the exact database or data directory it would have touched, so the refusal is checkable rather than assertable. Or an unclaimed failure of any of the three verbs — the clone connection refused, the data directory unwritable. X_BRANCH_EXISTS (the name is taken) and X_SQL_UNSAFE (the name is not [a-z0-9_-]+) reach you as themselves
Fix
for the refusal, x db branch list --json — the name is not in the set, and only what that lists can be deleted. Otherwise read cause, then x doctor --json