This task can be easily solved with brute-force.

Send possible flag. Endpoint will answer with the Levenshtein distance between your string and flag, that is amount of single character changes required to make one string from another.

1. Start with the flag template `spbctf{}`, and fill in as many chars as server will allow. Try using rare characters since common ones could be in flag.
2. Reduce number of filled in characters by one, notice the moment when distance starts growing again: it was right amount of characters on previous step.
3. Now brute-force every position with every printable ASCII symbol. Notice then distance will shorten, it would be correct character. Repeat with next one.
4. When you reach distance of zero, you'll have your flag.