In English, adjectives do not take a plural form. How do we explain the use of the adjective "sales" (as in sales growth, or sales decline)?

Solution 1:

According to Lexico, sale is not an adjective.

You are confusing a category of words adjectives with a function in noun phrase structure modifier. Modifiers in NPs can come from a variety of different categories. A selection from CaGEL p444:

i a. another three days b. the barely forty students present [Determiner Phrase]

ii a. his wry attitude b. many very angry farmers [Adjective Phrase]

iii a. the defeated army b. her recently published article [Verb Phrase]

iv a. the gleaming showroom b. three steadily melting marshmallows [Verb Phrase]

v a. its entertainment value b. those Egyptian cotton shirts [nominal]

vi a. a dogs’ home b. a young children’s edition [nominal]

Sales, as it is used in sales growth, is a mass noun (Lexico):

1 [mass noun] The exchange of a commodity for money; the action of selling something.

‘we withdrew it from sale’

1.1 (sales) A quantity or amount sold.

‘price cuts failed to boost sales’

1.2 (sales) The activity or business of selling products.

‘director of sales and marketing’