You had the "snap" function off. That way, when you cut a waveform, it doesn't cut at the end of a beat or bar, wherever you you click, meaning that even if you are very precise, eventually, you will get timing errors. Learn to always use the snap function when making samples to copy/paste.
What you could do to fix this problem with the sample you've already got is 2 things:
Retrack.
Zoom in really, really far, turn snap on, and clip off anything that if beyond the bar. If the waveform ends before it has reached the end of the bar, use the Draw tool to draw a silence until past the end of the bar, clip it off at the end (using snap, as alway) and glue the silence to the sample.
I'd retrack it, and use snap this time
It is the symbol near the right-middle of the top of the screen looking like this: >|<