I and several of my siblings lent another brother a significant amount of money through our family “gemach” to pay off the remainder of his car. We didn’t make a Pruzbu, and didn’t specify when he should return the moneyl. Can we still ask him to pay back the money?
Yes, there’s no problem in asking for the money back.
Shemitah only cancels debts at the end of the year (according to all but the Rosh’s opinion, and as ruled by normative halachic authorities). Therefore, you would only have a problem in claiming back the debt if you were to leave it over until the beginning of 5776.