As ERTE payments are unemployment benefits paid out by SEPE, they will be seen as another source of income from work for the purpose of your resident’s income tax RENTA in 2021, over tax year 2020. This could cause you to be obliged to file the RENTA income tax return, while in previous years you were not.
Also, you need to check if SEPE has withheld any IRPF retentions as when they have not, you need to be prepared for a different outcome of your RENTA declaration next year , then you might expect. The non-retained IRPF percentage will come up as still to be paid. You could set money aside, but you could also ask your employer to withhold a higher percentage of IRPF retentions from your wages, once back at work.
If you weren’t aware before and you will be on an extended ERTE till September this year, you can also ask SEPE to withhold the retentions, to limit the effect on your RENTA next year.