If it says REPLACE NOW do as your told and replace it. The battery is bad. Especially after 6 years of use, they degrade and lower capacity until they are completely dead.
You can confirm it using coconut battery. If it tells you it needs to be replaced, replace it.
MacBook Pro 15" Unibody Early 2011 Battery Replacement
Thats how to do it.
MacBook Pro 15" Unibody (Early 2011 through Mid 2012) Replacement Battery
Thats the battery. A-stock is about 30 dollars cheaper and it also is still a great battery for a macbook of this age. Its not really worth getting the new one.
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