Put this formula somewhere on the same sheet or another sheet and point your dropdown to the column where you place it.

```
=FILTER(
   SEQUENCE(50),
   COUNTIFS(
     A2:A1000, SEQUENCE(50),
     C2:C1000, "<>Done")=0)
```

# Explanation 
1. COUNTIFS is used in place of COUNTIF to allow adding the additional criteria `<>Done`.
2. SEQUENCE is used to create the list of numbers from **`1-50`**
3. FILTER removes numbers already in use based on the array of TRUE/FALSE values returned by COUNTIFS.