I would like to find a mode value in order to identify the most significant support/resistance line. I think that it is suitable for me rather than an average function or event median function.

For example, The below show LLV(L,20)
I need to find Mode value as like this Mode(LLV(L,20),100).
but I didn't find this function, It may be similar to median function https://www.amibroker.com/guide/afl/median.html

H, L, V are the default arrays but it's distribution function for any arrays.

What about looking into manual where there is clearly stated:

The function is general purpose frequency distribution function.

It is typically used to create distribution of volume versus price but this is only one of many possible uses.

In your case you have to replace array function arguments with LLV(..) or HHV(..) or whatever array you want to use. Note: priceH argument of PriceVolDistribution has to be larger than priceL argument.

As you can see in below example picture I am outputting most frequent LLV(...) since each new start of day. So variable "MODE" is yellow line.