मैं आज नहीं आ सकती, लेकिन मैं आपको संदेश भेज सकती हूँ।

Because the speaker is female.

In Hindi, forms like सकता / सकती agree with the gender of the subject:

• मैं ... सकती हूँ = said by a woman
• मैं ... सकता हूँ = said by a man

So if a man were speaking, the sentence would be:

मैं आज नहीं आ सकता, लेकिन मैं आपको संदेश भेज सकता हूँ।

सकती हूँ comes from the verb सकना, which means to be able to or can.

So:

• आ सकती = can come
• भेज सकती हूँ = can send

A useful pattern is:

verb stem + सकता / सकती + auxiliary

Examples:

• मैं जा सकती हूँ = I can go
• मैं लिख सकती हूँ = I can write

Because after सकना, Hindi normally uses the verb stem, not the full -ना form.

So:

• आना becomes आ
• भेजना becomes भेज

That is why you get:

• आ सकती
• भेज सकती हूँ

This is very common in Hindi:

• करना → कर सकता हूँ
• देखना → देख सकती हूँ
• लिखना → लिख सकती हूँ

In everyday Hindi, the present-tense auxiliary हूँ is often omitted, especially with forms like सकता / सकती.

So both of these are natural:

• मैं आज नहीं आ सकती
• मैं आज नहीं आ सकती हूँ

The version without हूँ often sounds a little more natural and conversational.
In your sentence, the first clause drops हूँ, while the second keeps it. That is completely normal.

In Hindi, नहीं usually comes before the verb phrase it negates.

So:

• नहीं आ सकती = cannot come
• नहीं भेज सकती = cannot send

In your sentence:

• मैं आज नहीं आ सकती = I cannot come today

If you wanted to negate the second action instead, you would say:

• मैं आपको संदेश नहीं भेज सकती हूँ = I cannot send you a message

आपको is made of:

• आप = you
• को = a marker often used for an indirect object or recipient

So आपको means to you.

Since the message is being sent to someone, Hindi uses को:

• मैं आपको संदेश भेज सकती हूँ = I can send a message to you

Also, आप is the polite or formal word for you.
Less formal versions would be:

• तुम्हें = to you, informal
• तुझे = to you, very intimate or very informal

Yes. The word order is very normal for Hindi.

Hindi often follows Subject–Object–Verb order, unlike English.

A rough breakdown is:

• मैं = I
• आज = today
• नहीं = not
• आ सकती = can come
• लेकिन = but
• मैं = I
• आपको = to you
• संदेश = message
• भेज सकती हूँ = can send

So the literal order feels more like:

I today not come can, but I to-you message send can

That is just how Hindi naturally organizes the sentence.

It is repeated for clarity and natural rhythm.

Hindi often repeats the subject when starting a new clause after a word like लेकिन.

So both are possible:

• मैं आज नहीं आ सकती, लेकिन मैं आपको संदेश भेज सकती हूँ।
• मैं आज नहीं आ सकती, लेकिन आपको संदेश भेज सकती हूँ।

Both are correct. Repeating मैं just makes the contrast a little clearer.

लेकिन means but.

It connects two contrasting ideas:

• cannot come today
• can send a message

Other common Hindi words with a similar meaning are:

• मगर
• पर

All three can mean but, though लेकिन is very common in neutral standard Hindi.

संदेश is correct and natural, and it means message.

However, in everyday modern speech, especially for phone or text messages, many speakers also say:

• मैसेज

So these are both possible:

• मैं आपको संदेश भेज सकती हूँ।
• मैं आपको मैसेज भेज सकती हूँ।

संदेश sounds a bit more standard or formal.
मैसेज sounds more everyday and conversational.