Most of the time when I try to put a video in it says / not seek able. Why?
Most videos are seekable, where are you getting your videos from? You may be able to tweak the parameters of the video creator to make them more seekable.
My understanding is that Shotcut (and other editors and some vdeo players) uses I-frames when seeking. For an explanation of I-frames see here (GOP, B Frames and Codec Threads - What Do They Mean?)
If the video’s I-frames are sparsely scattered through the video (i.e. there is a very high ratio of B- and P-frames to I-frames, it makes it very difficult to seek on the video.
For such videos Shotcut warns you that the video is non-seekable, and gives you the option to create one that is, that you can edit. Basically it copies all the frames of the video to a new one that that has a much better GOP to make seeking possible.
Usually this occurs because Shotcut was not able to detect a standard frame rate for your video and then assumes that it might be variable frame rate. Errors are common when trying to seek in variable frame rate videos because it is not possible to determine a frame position without scanning the entire file from the beginning for each frame.
Your file may or may not actually be variable frame rate. Shotcut is just warning you because the detection was not conclusive. If your file comes from a camera or smart phone, it will probably seek just fine. I have one phone that records videos that always trigger the warning in Shotcut. I just ignore it and edit the video like normal - I have been happy with the results.
My advice is to try a “test” edit with your files and see if you are happy with the results. If you export a test clip and everything plays smoothly, then it will probably work fine and you can ignore the warning.