@Zeina, I had this issue a while ago when I first started using Qiime2. Basically, your computer is running out of memory while Dada2 is denoising the sequences. This can happen if you are trying to work on a large dataset, or your computer has a small amount of RAM. How are you running Qiime2? If it is through Docker, you may need to give it more of your RAM to use. If not, you may have to find a different computer to analyze your data with.
Alternatively, you could break your data up into smaller chunks and analyze a handful of samples at one time (instead of all at once). The authors of Dada2 claim you can do this without losing meaningful sensitivity, but I’m not sure if this has been directly tested. Someone with more working knowledge on this particular topic is welcome to correct me.