Why does my battery only work for 15 min?

I bought IFIXIT whole battery replacement Kit and successfully replace my MacBook Pro laptop 6 months ago. The Cycle count is only 136 but after fully charged to100%, I can only use it 15 min and it dies after. I can’t seem to use it without plugging the charger the whole time. The “Service Battery “ icon shows on the top status bar. I don’t understand why? I took it to Apple Store they run diagnostic on it and they told to me the battery is not good “I need to replace new battery“. I don’t understand why? does this mean IFIXIT sold me a bad battery? Did any one had similar experience as mine?