The answer will always be either a repdigit (a number composed of repeated instances of the same digit) or a power of 10. We find the smallest power of 10 that cannot be expressed, and for each digit we find the smallest repdigit that cannot be expressed, and take the smallest of these.
@shubh_ism310, think about the case when zero and another digit has same frequency and this frequency is also the minimum frequency amongst all 10 digits.