Why can't I reserve seats on Lufthansa prior to check in?

Interesting.....

Usually LH will at least give you the option of reserving a specific seat if you pay a fee. Did
you even get that choice?

I believe that people with status can pick their seats for free. I have UA status and was able
to select a business class seat on LH for no charge (it was booked with miles, and that may
make a difference).

Did you buy your ticket from Lufthansa or from a partner like United?

https://www.lufthansa.com/us/en/advance-seat-reservation

I flew Lufthansa LAX => Budapest via Munich in December - I was able to reserve my premium economy seats in advance by paying the appropriate fees.

No option to reserve seats for a fee. Seats are included in business class.
We did not use miles. We paid for the tickets.
We do not have status with LH.
Neither do our friends who flew from EWR or the Swedes who did get seat choice.

On the surface, it doesn't make sense, which probably means there is something going
on that you're not aware of.

You do mention "codeshare" in your original post. If you booked through a partner airline
on LH metal, you would get a UA locator code for your booking, AND a LH locator code.
That is what you would use (on lufthansa.com) to reserve your seats.

But that doesn't explain why you were able to reserve seats on the intra-Europe leg....

I had codes for both LH and UA. I used them both in the respective websites to attempt to reserve seats. So did our EWR friends. The UA website would immediately say we had to reserve through LH. LH would say we had to wait for check in. We are all very experienced travelers, not newbies. This is the first time this has happened to any of us. Our friends did have status with UA. (I just asked them again. I thought they didn't. ) It did not help them.

And you are right. Why could we reserve seats for the intra-Eueope flight but not for the overseas?
I am hoping someone can make sense out of this.

We booked Lufthansa round trip from Boston to Ljubljana in December and flew the end of March, returning two weeks later. When I booked the trip the page seemed different than the last time we had flown Lufthansa and I did not easily find where to reserve the seats at booking. I did find them by scrolling farther through the page. A friend booked a few weeks later and mentioned they could not find where to reserve seats during booking and continued without doing so. When I mentioned what I found, they went back in and did find where the could reserve them without waiting for checkin. Whether something else has changed since our December booking I am not sure.

I am afraid it was flight-specific. There was a place to reserve the seats for FRA-BCN and BCN-FRA, bUt there was no place on the exactly similar page for IAD-FRA ot FRA-IAD.

Did you make your booking through United or directly with Lufthansa?

I recently flew a codeshare on LH, booked through UA. I booked a seat in Premium Economy from ATL-FRA (which was just average, BTW) and was unable to choose a seat until check-in. Luckily, when check-in opened, there were plenty of seats. They had assigned me one, but I was able to change it with no issues. I don't recall if I was able to choose a seat for the next leg, but as it was barely more than an hour, I didn't worry about it, and I wound up having a whole row to myself anyway.

I think it's pretty common to not be able to choose seats on a flight operated by a codeshare partner. I have had this same issue with Delta and KLM.

I reserved through LH but gave them my UA frequent flyer number. I could reserve seats for the second leg, but not for the first.
I used the UA code share number to try through the UA website, but got the same answer: Seat assignment not available until check in. I actually changed our FRA-BCN seats once on the UA website to see if that would work and it did it with no problems.

I wonder if it would make a difference if you book your itinerary through Lufthansa (which operates the EU domestic flight) or through United (which operates the transatlantic international flight). I usually book my tickets through the airlines that operates the transatlantic international flight.

Edit:
The intercontinental flight is the longest and the most important flight (typically only one daily flight) while the EU domestic flight has many more flights for rebooking if needed. One likely will have no problem preselecting seat on the longest leg of the flight if you book through them directly; also they likely will have direct closer tracking of your checkin luggage.

See if this applies to your flight:
https://businesstravelerusa.com/news/lufthansa-business-class-new-seat-charges/

It indicates in their new ”Allegris”business cabin only 12 of the seats can be booked for free in advance.

Otherwise, I would suggest you ask on Flyertalk.

It was not the Allegris setup. We flew on an older plane.
I will check Flyertalk. Thanks.