'Difficulty' can often be used as either a countable or uncountable noun, with no change in meaning. For example:

I had a lot of difficulty accessing this site.

I had a lot of difficulties accessing this site.

In the first sentence, 'difficulty' is uncountable, while in the second 'difficulties' is countable. The meaning is the same. In many cases, it doesn't matter which form you use.

I suspect that the trouble you've been having is getting the grammar of the rest of the sentence to match up with the form of the word 'difficulty' i.e. whether you're using it as a singular or plural noun. For example:

I had many difficulty

I had much difficulties

I had a few difficulty

Countable nouns have sigular and plural. Example: flower, table, hand (that is sigular) and in plural it will be: flowers, tables, hands.

Uncountable do not have plural. Example: water, sky, Earth, money

In questions for countable nouns we use How many, and for uncountable how much.

That is just basic, hope it helps. :))

Oh, after Su.Ki. comment I see you asked about noun <em>difficulty</em>. I did not read well and I thought you said that you have difficulty with understanding countable and uncountable nouns.

I am so sorry...

https://learnenglish.britishcouncil.org/en/grammar-reference/countable-uncountable-nouns-1

Thank you Su. Ki/ Does it mean that it doesn't matter which form I use plural or singular?