Can you please provide a REPRoducible EXample of your problem? It provides more specifics of your problem, and it helps others to understand what problem you are facing.
In case you don't know how to make a reprex, here's a great link:
Also, please include a sample dataset and the errors you're facing while trying return(mp_fmp[delta_fr != 0 & f_vol.t1 > 0]).