No Battery, X over Battery and slowness?

Hi, so I have a Mid 2012 MacBook Pro. It’s a friends to whom I’ve been asked to look at it. It will turn on completely fine plugged into the MagSafe charger. But it doesn’t even detect a battery even with the side battery indicator button. It has a green light on the MagSafe but the battery indicator is a battery with an x in the middle. I tried resetting the SMC to which I’m not sure I was 100% successful at. I also opened up the laptop, disconnected and reconnected the battery , and it din’t do anythhinnf.

But I reinstalled the os and wiped the Hard drive. If the battery itself is the problem I have no issue replacing it myself, I’d just not rather spend the money on the battery and it not be a problem. Let me know what you guys think!

Also the laptop is a little slow, would it be better to replace the Hard drive with an SSD or the Ram or both?

Okay awesome! I will do that. So I did run coconut battery before I just forgot to add that in. It just says N/A or 0 for everything. See picture.