Hi There,I started a new business 6 months ago. So far, I have accumulated about $5000 in sales tax e.g. GST/HST, Can I offset this against a loss of $10k for the FY?

Not clear what you mean - but the answer is certainly NO regardless.
If you are registered for GST and collect it from customers; that has to be reported separately from the rest of the business. Whether you have to pay any of it to CRA depends on your specific situation.
But it has nothing to do with earnings (or loss)