Hey! Unfortunatly I don't have experience with this one, but I did find the manual to talk about quantizing items as shown below:
https://dlz.reaper.fm/userguide/ReaperUserGuide641c.pdf
Page 166:
9.16 Quantizing Items
REAPER allows you to quantize your audio items. This essentially means lining them up according to the project
tempo or time signature.
You might have a situation in which you have copied and pasted a sample several times within a track (or across
several tracks) and you need to ensure that they are correctly aligned. To ensure that all items are aligned to the
beat, you simply do this:
1. Double-click on the track in the Track
Control Panel to select all the media items
in the track. Alternatively, you could right
click drag with your mouse to marquee or
"lasso" them.
2. Right click over any of the items, then
choose Item Processing then Quantize
item positions to grid... This causes the
Quantize Item Positions dialog box to
be displayed.
3. Specify your required parameters,
including the note length setting (1/8, 1/4,
1/2, 1, 2, etc.) and whether you want the items stretched to fit. An example is shown on the right.
4. Click on Process for the quantizing to be done.
In the example above, the option to also quantize item ends and stretch to fit was enabled.