Questions & Answers about Należność jest wysoka.
Należność is a noun, and it is feminine.
That matters because adjectives in Polish must agree with the noun in gender, number, and case. Here, since należność is feminine singular, the adjective appears as wysoka.
This noun is often used in formal, financial, accounting, or legal contexts. It can mean something like an amount due, a receivable, or a sum owed, depending on context.