So I need to get a brand new battery is the solution?
If the battery still gives you the range you need, then just keep using it. Just be aware that you'll probably need to replace it sooner rather than later.
It's only theoretically knackered; the SOH is based on cycle count, i.e. the number of times it's been fully charged and discharged, often based on a range of between 500 and 800 times depending upon the cells used. That's expected life, not actual.
State of Health (SOH) is just an indication and not an accurate measure of actual battery condition - the BMS is just making assumptions based on count. If the battery has been well looked after (not left overcharged or over discharged for periods of time and not hammered or rinsed by excessive current demands), it may go on for a good while yet.
It might be an idea to check its voltage once it's fully charged. It should ideally be between 41.5 and 42v. Much less than that usually suggests it's near end of life. That's probably a better indication.