This is not causing your error, but you should not merge multiple runs prior to running through dada2. dada2 must be run on each run separately to properly model the run error. See this tutorial for an example of merging multiple runs after dada2.
Your error appears to be here:
Make sure you have the correct forward/reverse files, and that both runs are paired-end (if not, scrap the reverse reads from the other run and analyze as single-end data).
I hope that helps!