Welcome to the forum, @iratxez !
@wburgess's pointer may not have solved your issue, but it was sound advice. Once you resolve the error you're dealing with, be sure to stick with the same DADA2 parameters whenever you're merging after denoising. 
This error message indicates that some of the Feature IDs in your table don't exist in the taxonomy you're using. This could happen if you used a taxonomy built with some but not all of your features (e.g. if you built the taxonomy before merging), if your taxonomy was built from a filtered set of sequences (I've done this!), and occasionally in other fun ways.
Please take a look at the provenance tab of your QIIME 2 artifacts in q2view and see if you can puzzle out what went wrong. If you can't figure it out, please share the artifacts you're using as inputs when you get this Error message, and we'll figure out it out together.
Best,
Chris