Money symbols are limited to 12 characters. I see that your GB00BYY5F144 is 12 characters.
======dealing with reuse of a deleted symbol==============
Symbol reuse happens, but if you "deleted" the investment, you will not find it listed in the Portfolio, even if you select to show closed positions.
If you really want to get rid of symbol GB00BYY5F144 for reuse, try entering a dividend for GB00BYY5F144 into the name of the investment; type those 12 charactersers, and then hit Tab. Choose Dividend. Note that the name in the box may have changed. Finish creating the dividend. (I have only tried with shorter symbols)
Right-click that dividend transaction, and choose GoToInvestment. Click Details. Whatever that investment is, if the symbol is GB00BYY5F144, blank
or change its symbol. If the name is GB00BYY5F144, Rename it to something else.
The symbol GB00BYY5F144 should now be freed up. Delete the fake dividend.
This is also the method to undelete an investment.