How can I convert the negative values of the ERA5 PET data into positive?
You can multiply all the values by -1

cdo mulc,-1 infile.nc outfile.nc


If you want also to get rid of the condensation data (and, please, note that I am not implying that that is the right thing to do), you can do something like this

cdo setrtoc,0,9999,0 ifile.nc ofile.nc


Hope this helps