That's strange. I wonder what's going on. At first, I thought it might be a space issue, but no, I have 29GB free. I have auto updates on and this is a older Pro my son uses, I thought it would have updated on it's own by now, since 13.3 came out over a month ago.
Had the same issue with a brand new iPhone 11 pro, unlocked yesterday. Factory loaded with 13.2.2 and wouldn't update OTA. Finally was able to do manually through iTunes with help from Apple Tech Support. They "claim" engineers are looking into it. Appears to be a thing.
I have the same problem with an AppleTV HD. 13.2 is installed and the update to 13.3 shows the mentioned error every time. I can't even load the IPSW via ipsw.me. Seems something is wrong with Apples update servers. Strangely enough, I can download nearly every other IPSW for various devices, except tvOS 13.3 for the ATV HD. I get the mentioned error since thursday, on wednesday I updated another ATV HD to 13.3 without any problems.
I also can't update via finder (ATV connected over USB-C), same error ...