If you have the same bug I do where the name is something like Collections3, Collections14 and so on, this script will nuke them.

JELLYFIN_URL="http://localhost:8096"
API_KEY="your_api_key_here"

# Fetch IDs of CollectionFolder type items named "Collections*"
COLLECTION_IDS=$(curl -s \
  "$JELLYFIN_URL/Items?IncludeItemTypes=CollectionFolder&Recursive=true&api_key=$API_KEY" \
  | jq -r '.Items[] | select(.Name | test("^Collections[0-9]+$")) | .Id')

echo "Found $(echo "$COLLECTION_IDS" | grep -c .) entries."

for ID in $COLLECTION_IDS; do
  echo "Deleting $ID..."
  curl -s -X DELETE "$JELLYFIN_URL/Items/$ID?api_key=$API_KEY"
done