It’s because you tried to update a table without a WHERE that uses a KEY column.
The quick fix is to add SET SQL_SAFE_UPDATES=0; before your query :
SET SQL_SAFE_UPDATES=0;
Or
close the safe update mode.
Edit -> Preferences -> SQL Editor -> SQL Editor
remove Forbid UPDATE and DELETE statements without a WHERE clause (safe updates) .
BTW you can use TRUNCATE TABLE tablename;
to delete all the records .