Unable to determine if battery is a genuine part -- Is it a problem?

You can indeed ignore the error message; the functioning of the battery is not affected in any way by it. It's my understanding that the Repair Assistant will only allow you to "Restart and Finish Repair" if you are using a genuine Apple part; no aftermarket parts are supported by it. Since I'm about 90% sure iFixit doesn't sell genuine Apple parts, you're not going to be able to get that message to go away with that battery.

The only other consequence you'll see as a result of using an aftermarket battery is that you will be unable to view the health information; that's the percentage of full capacity the battery is capable of. Generally we know to replace batteries when they get below about 85%, but unfortunately you won't have that to go by.

Outside of using a genuine Apple part, the only other way to get that message to go away is to save the BMS, or Battery Management System, from the original battery and transplant it onto a new cell. That's the little tiny circuit board that's mounted on the battery cell's positive and negative terminals and provides battery information to the phone such as temperature, current and voltage readings. You can buy kits that include the bare cell and some tape, but you'll need a soldering iron or preferably a spot welder to make that change. This is a deliberate attempt by Apple to make self repair as difficult and unattractive as possible in order that they get the money for those common repairs.