In The Merchant Of Venice, why couldn't Antonio give Bassanio any money?

Antonio could not give Bassanio any money since he did not have any ready cash available, as he explains to him:

Thou know'st that all my fortunes are at sea;
Neither have I money nor commodity
To raise a present sum:

What he means is that all his money has been invested in transporting commodities via ships to various destinations, since he is a sea merchant. Antonio also declares that he does not have any products which he can sell to raise the money Bassanio so desperately needs.

However, since Antonio wants to help his friend, he offers him an alternative:

therefore go forth;
Try what my credit can in Venice do:
That shall be rack'd, even to the uttermost,
To furnish thee to Belmont, to fair Portia.
Go, presently inquire, and so will I,
Where money is, and I no question make
To have it of my trust or for my sake.

Since he is a respected businessman, Antonio advises Bassanio to use his good name and creditworthiness to establish what kind of money he can raise in Venice. He tells Bassanio to try his utmost and do whatever he can so that he may have enough cash to go to Belmont where he could chance his luck at winning the beautiful Portia's hand. He tells him to leave soon and inquire where he can find money (a loan). He will also do his own enquiry to find out who would be prepared to extend him the finances Bassanio needs. He gives Bassanio the assurance that he will not question the issue about the sum being secured through the use of his name, or for his sake.   

It is this unselfish act by Antonio which forms the basis of his relationship with Bassanio. He is prepared to do everything that needs to be done to assist his friend. It is, therefore, unfortunate that his magnanimity is exactly what gets him into trouble with the moneylender, Shylock.  

Antonio is able to secure an interest-free loan of three thousand ducats from Shylock for Bassanio's use. The loan has to be repaid within three months with the condition that, if Antonio should forfeit, he has to allow Shylock to cut out a pound of his flesh. Antonio agrees to these terms and accordingly signs the bond, much against Bassanio's appeal that he should not. Antonio is confident that he will easily be able to settle the debt even before the due date since his ships would have returned after delivering their cargo and he will have more than enough money. 

Ironically, destiny intervenes and Antonio's plans go horribly awry. His ships are destroyed at sea and he cannot settle the debt on the due date. Shylock sees this as an opportunity to take revenge against Antonio whom he despises. Fortunately, Bassanio's newly wed bride, Portia, intervenes and ultimately, it is Shylock who is punished and Antonio is absolved of his debt.