When on a full-time ERTE, during the time you are covered by the ERTE, you do not build up any holiday entitlement as you only do so when there is a work relationship. When receiving full-time ERTE payments, there isn’t one.
So your total minimum entitlement for 2020 of 30 calendar days or 22 workdays, will be less.
If you were on a part-time ERTE, e.g. you were working 50% of your regular hours every workday, then your holiday entitlement will not be less, as you still had a work relationship throughout the ERTE, but your holiday pay will be 50% for the pro-rata part of the year you were on ERTE, i.e. the days you built up during that time.
If you had a holiday planned and obtained permission for, but couldn’t go because of the State of Alarm, then those days should be cancelled and not considered as consumed.