Question

Looks like there is something wrong with the battery and the iPhone can’t determine its capacity.

Did you recently have the battery replaced or something?

You may need to take in in to Apple to get it diagnosed and repaired if necessary.

If you mean the maximum capacity compared to when it was new go to Settings/General/Battery Health.

If you mean its design capacity in Milliampere-Hours (MaH), Apple doesn’t publish that information but you can find it with a web search, and, if you have a Mac, you can download the Coconut Battery app, which will display battery information for a plugged-in iOS device.

Go to Settings->Battery->Battery Health.

What was not found?

That means your battery was replaced with a 3rd party (non-Apple) battery. Only Apple batteries can report maximum capacity, because the circuit that does that is built into the battery.