isnt enough to buy a ship ?

I think having over 2 billion credits in cash corrupts your character/save game, but I am not sure.

Put some money in the bank (about half of what you have) and try again.

Money on your character is saved as a 32-bit signed integer value. Maximum possible value is 2,147,483,647.

You are very close to the maximum, and if you add any more, because of the way integers work, it rolls over to its minimum value of -2,147,483,648, which is just a bit short for any purchase you'd like to make.

Quote from Apache

Put some money in the bank (about half of what you have) and try again.

Um. This is SP, apparently.

I think the solution is to edit your save file. I have no other idea,

Try using Freelancer Save Editor (first post) to lower your credits amount to ~ 1B.