not found ...

Sorry, STRING did not find a protein called 'P20039'.

Generally, STRING understands a number of different names/symbols for proteins.
Here is a selection of typical names: 'YEL036C', 'TRPB_ECOLI', 'trpB',
'ENSP00000249373', 'BRCA1', 'CG11561', 'daf-3', ...

Try to identify your protein with a different name/symbol that you might know.
Alternatively, you can paste the raw amino-acid sequence of your protein
into the input-form, and STRING will try to identify it via a similarity search.