Проверьте кадастровый номер не позднее чем сегодня.

Проверьте is the perfective imperative of проверить.

In Russian, the perfective imperative usually means do this once / complete the action. That fits this sentence, because the speaker wants one completed result: check the cadastral number.

• Проверьте = check it / make sure it gets checked
• Проверяйте = keep checking / check regularly / be checking

So here:

• Проверьте кадастровый номер... = Check the cadastral number...
• Проверяйте кадастровый номер... would sound more like a repeated instruction, for example in a manual: Always check the cadastral number...

The ending -ьте marks the imperative in a polite or plural form.

Russian imperative forms can distinguish:

• Проверь = singular informal, said to one person you address with ты
• Проверьте = plural or polite, said to:
  • more than one person, or
  • one person formally, with вы

So this sentence is either:

• a polite instruction to one person, or
• an instruction to several people

This is very common in notices, official instructions, customer service language, and written requests.

It actually is the direct object, but because of the type of noun, the form looks the same as the nominative.

The verb проверить takes a direct object in the accusative case.
Here the object is:

• кадастровый номер

For inanimate masculine nouns, the accusative singular is usually identical to the nominative singular:

• nominative: кадастровый номер
• accusative: кадастровый номер

So even though the form does not change visibly, grammatically it is the accusative direct object.

Кадастровый is an adjective modifying номер.

It comes from кадастр and means something like cadastral / land-registry-related.
It agrees with номер in:

• gender: masculine
• number: singular
• case: accusative (same visible form as nominative here)

So:

• кадастровый номер = adjective + noun
• both words match each other grammatically

This kind of adjective-noun agreement is very important in Russian.

Не позднее чем means no later than.

Word by word, it is roughly:

• не = not
• позднее = later
• чем = than

So literally it is something like not later than.

This is a fixed comparative-style expression used with dates, times, and deadlines:

• не позднее чем завтра = no later than tomorrow
• не позднее чем в пятницу = no later than Friday
• не позднее чем через неделю = no later than in a week

It is fairly formal and very common in instructions, regulations, contracts, and official writing.

Because сегодня works perfectly well as an adverb of time after не позднее чем.

So:

• не позднее чем сегодня = no later than today

Russian often uses a simple time word after this expression.

That said, in more official or bureaucratic language, you may also see variants like:

• не позднее сегодняшнего дня
• не позднее конца сегодняшнего дня

These sound more formal or more precise.
The version with сегодня is shorter and still understandable.

Yes, it is understandable and acceptable, especially in formal instructions, but it sounds a bit bureaucratic or official.

Depending on context, Russian speakers might also say:

• Проверьте кадастровый номер сегодня.
• Проверьте кадастровый номер не позже сегодня.
• Проверьте кадастровый номер до конца дня.

The exact choice depends on what is meant:

• сегодня = at some point today
• до конца дня = before the day ends
• не позднее чем сегодня = a deadline-style phrase, emphasizing not after today

So the original sentence sounds most like an official instruction or written notice.

Yes, Russian word order is flexible, but the original order is the most straightforward for an instruction.

Original:

• Проверьте кадастровый номер не позднее чем сегодня.

Possible variations:

• Не позднее чем сегодня проверьте кадастровый номер.
• Кадастровый номер проверьте не позднее чем сегодня.

These are all grammatical, but they shift emphasis:

• starting with Не позднее чем сегодня emphasizes the deadline first
• starting with Кадастровый номер highlights what exactly must be checked

For a neutral instruction, the original order is the clearest.

Because не позднее чем сегодня is just a time expression attached to the main clause, not a separate clause.

There is only one finite verb here:

• Проверьте

Everything else is part of the same instruction:

• what to check: кадастровый номер
• by when: не позднее чем сегодня

So no comma is needed.

Yes, depending on context.

The Russian phrase не позднее чем сегодня most directly means:

• no later than today

In natural English, that may become:

• by today
• by the end of today
• today at the latest

Which English version is best depends on how strict the deadline is.

In Russian, if the speaker specifically wanted by the end of today, they might also say:

• до конца дня
• не позднее конца сегодняшнего дня

So the original sentence gives a deadline of today at the latest, but it does not spell out the exact final moment as explicitly as some longer versions do.

You would usually say:

• Проверь кадастровый номер не позднее чем сегодня.

Here:

• Проверь = singular informal imperative
• Проверьте = polite/plural imperative

So the only change is the verb form.

Compare:

• Проверь... = check... (to one person, informal)
• Проверьте... = check... (to one person formally, or to several people)

The usual stress is:

• Прове́рьте када́стровый но́мер не по́зднее чем сего́дня.

Word by word:

• прове́рьте
• када́стровый
• но́мер
• по́зднее
• сего́дня

This can help a lot, because Russian stress is unpredictable and must often be learned word by word.