Some offerings have minimums and some don't. If the software won't allow you to enter a number below a certain amount then the amount you are trying to enter is below the minimum set by the issuer for that offering.
Minimums can come in two forms, “soft minimums” and “hard minimums”.
If a Deal has a “hard minimum” you will not be able to invest an amount less than the hard minimum. If a deal has a "soft minimum" you will be able to invest below the minimum but the issuer may not accept the investment.
Click here for an article on the fastest way to determine if your deal has a minimum.