Learn Italian: La scadenza è improrogabile, altrimenti perdiamo il contratto. - The deadline is strict, otherwise we lose the contract.

Not automatically. Altrimenti simply means "otherwise" and takes whatever mood fits the meaning of the consequence. Here, it takes the present indicative (perdiamo) because losing the contract is presented as a certain, factual result. You would only use the conditional if the consequence were hypothetical (for example, altrimenti perderemmo il contratto - "otherwise we would lose the contract").

Could I say sennò instead of altrimenti in this sentence?

Yes, sennò (or se no, meaning "if not") is a perfect synonym for altrimenti. However, altrimenti is slightly more formal and professional, which makes it the better choice for this specific business context involving a deadline (scadenza) and a contract (contratto).

Improrogabile is translated as "strict" here. What is its literal meaning?

It literally means "unextendable" or "cannot be postponed." It is built from the noun proroga (an extension or postponement), the negative prefix im-, and the suffix -abile (-able). It is the most precise vocabulary word in Italian for a "hard deadline."

Since losing the contract is a future consequence, why use the present tense perdiamo instead of the future perderemo?