Procena štete od poplave je spremna.

Because štete is in the genitive singular.

In Serbian, after a noun like procena meaning assessment or evaluation, the thing being assessed usually goes in the genitive:

• procena štete = assessment of damage
• procena rizika = assessment of risk
• procena situacije = assessment of the situation

So:

• šteta = damage, damage/loss
• štete = of damage

This is a very common Serbian pattern where English uses of.

Here is the breakdown:

• procena — nominative singular, feminine
  It is the subject of the sentence.

• štete — genitive singular
  It depends on procena: assessment of damage

• od poplave — genitive singular after the preposition od
  poplave is genitive singular of poplava

• je — present tense, 3rd person singular of biti = to be

• spremna — feminine singular adjective, agreeing with procena

Od + genitive often expresses source, cause, or origin.

So šteta od poplave means:

• damage from a flood
• damage caused by a flood

In this sentence, od poplave modifies štete, not procena. The structure is:

• procena
  of štete
  from poplave

So the meaning is the assessment of damage caused by the flood is ready.

Because spremna agrees with procena, which is a feminine singular noun.

In Serbian, adjectives must agree with the noun they describe in:

• gender
• number
• case

Since procena is feminine singular nominative, the adjective must also be feminine singular nominative:

• spreman — masculine singular
• spremna — feminine singular
• spremno — neuter singular

So:

• Procena je spremna. = The assessment is ready.

The sentence is built around procena as the main noun:

• Procena ... je spremna.

That gives you the basic frame:

• The assessment ... is ready.

Then štete is attached to procena as a dependent genitive:

• procena štete = assessment of damage

So štete is not the subject here. It is part of the noun phrase that tells you what kind of assessment it is.

A useful clue is that procena is in the nominative, while štete is in the genitive.

Yes, in a normal full sentence, je is needed.

Serbian usually keeps the present-tense form of to be in sentences like this:

• Procena je spremna.

Unlike in some Slavic languages, you do not normally leave it out in standard prose.

You may sometimes see omission in:

• headlines
• notes
• very compressed style

But in an ordinary sentence, je should stay.

Yes. Serbian word order is flexible, although the original version is the most neutral:

• Procena štete od poplave je spremna.

Other possible orders include:

• Spremna je procena štete od poplave.
  This puts more focus on ready.

• Procena je spremna.
  Shorter, if the rest is already understood from context.

The original order sounds natural and straightforward for giving information.

No. Serbian does not have articles like English a and the.

So poplava can mean:

• a flood
• the flood

and context tells you which one is meant.

That is why od poplave can naturally mean either:

• from a flood
• from the flood

In real use, the situation usually makes it clear.

Yes, that is also natural.

Compare:

• procena štete od poplave = assessment of damage caused by a flood
• procena poplavne štete = assessment of flood damage

The second version is more compact because poplavne is an adjective meaning flood-related.

Both are good. The version with od poplave may sound a little more explicitly tied to damage resulting from the flood itself.

Yes, they are the same word in different standard variants:

• procena — Ekavian
• procjena — Ijekavian

So depending on region, you may see:

• Procena štete od poplave je spremna.
• Procjena štete od poplave je spremna.

Both mean the same thing.

Possibly, but the nuance changes a bit.

• spremna = ready
• gotova = finished, done
• završena = completed

So:

• Procena štete od poplave je spremna.
  The assessment is ready, available, prepared for use.

• Procena štete od poplave je gotova.
  The assessment is done.

• Procena štete od poplave je završena.
  The assessment has been completed.

In many contexts they are close, but spremna highlights readiness more than completion.