I'm trying to add Lookup field to the Mistakes table to get the Currence definition stored at the Mistakes
I'm afraid that doesn't make sense. You are looking up a value from Mistakes to Mistakes?
If you need to see data from Trades on a layout based on Mistakes, you can simply add the needed fields from Trades to your layout.
Sorry, want to look up a value from Trades, not Mistakes.
I can add the field from the Trades to the layout, but I want to create a Mistakes report sorted by Currency (lookup). Is it possible to get sub-summary based on the field from related table (Trades)?
Yes, you can select the related field to make it part of your sort order and you can select it as the "when sorted by" field in your part setup.