An image is actually not very useful. The best thing to post is a Reproducible Example (reprex). Here is an explanation of reprexes:
I suggest you show us some small data frames which have the crucial columns from your xlsx file and the .txt file that contains the mapping from FBpp ID to FBgn ID. If I understand your explanation, the examples only need to have a few rows. I am not sure if the gene symbol is in the same file as the ID mapping. If it is not, we will need an example of that also.