Key won't come out after transmisión swáp from automatic to manual

You likely affected the key interlock system, when you did the swap. Review how things were set up before and now. Also, you want to check all under dash and under hood fuses with a test light or multi-meter.

Not knowing what kind of car it is I have to ask. Since you were switching from automatic to manual was there a switch on the linkage so the car knew it was in "Park" ?

If it is a modern car it may need to be reprogramed to function with a different style transmission.