‘effort’ can be countable or uncountable.

Uncountable (= the physical or mental energy that is needed to do something)

e.g. Learning a language takes considerable effort.

Countable (= an attempt)

e.g. Please make an effort to help.

Get more examples here.

Plural IS possible e.g.

‘Despite all our efforts we lost the match’.

But the expression ‘make an effort’ is singular.

Common mistakes:

We need to make efforts to improve the situation.*

We need to make an effort to improve the situation.