The backrest and the seat pan need to be lined up perfectly, and the backrest should be inserted into the end of the groove.
If you failed to insert the screws into one of the sockets, it is most likely that the backrest and the seat pan didn't line up properly.
Please check out the instructions below and try assembly once more.
1. Please disassemble all of the chair parts.
2. Put the chair parts on the box or a desk as you can find in the picture in the attachment, and try to assemble them again.
3. Insert the bolts and screw following the numbered spots in the picture.
4. Before screwing the bolts, please make sure to press the joints very hard.
(In case it doesn't work, it is helpful to put up the seat side with a hand and push down the backrest while inserting the bolts.)
It would also be helpful if you look up the 'T50 Assembly' video on YouTube.