Black plays from 7th and 8th rank, his king starting from e8, so the white pawn is on b7.
There is no (FIDE) rule that forbids a combination of taking and promotion. b7xa8D (or whatever) is a legal move.
When playing online at some minor platform, you will often find that the rules of chess are badly implemented. I know of no single platform that plays ...
The move is legal.
If it were blacks move the computer would block it.
If White were in check the computer would too.
LIke @Glorfindel said the popup probably asked for which piece so blocking it stopped the move. But in that case you should still not have been able to push straight instead of capturing.
Do tell us what program/website that was!
I do not ...
Since the board is mirrored, the pawn is on b7 and the rook on a8. But b7xa8 promoting to a piece is a valid move.
I'm not sure which program/website this is, but it could be that a popup is asking to which piece White wants to promote, and something/your browser is blocking this popup, and your friend cannot complete the move.