The centre-back was handed a two-match ban by UEFA for “clearly receiving a yellow card on purpose” after he was booked in the 89th minute of Madrid’s last-16 first-leg tie against Ajax with his team leading 2-1.
Real Madrid without Ramos subsequently crashed out of the competition with a shock 4-1 second-leg defeat to the Dutch side at the Bernabeu.
“Was the yellow card in Amsterdam an error? Absolutely it was an error and I take the blame 200%,” Ramos wrote on Twitter.
Amid a difficult campaign at the Bernabeu, Real Madrid confirmed the reappointment of Zinedine Zidane on Monday, who replaces stand-in coach Santi Solari.
Madrid are third in La Liga and 12 points behind leaders Barcelona, who have beaten them in the league and cup since the end of February.
"As footballers, we like to do our talking on the pitch, but this season is not turning out that way," Ramos added.
"Recent events have been disastrous and I'm not hiding. We are not hiding. We the players are primarily responsible and I, as captain, more than anyone.
“These reflections are, without doubt, the result of a deeply disappointing season but if success didn’t stop us, we're not going to let defeat stop us. It's our obligation to carry on, to work and to evolve.
“And to remember that some of us are lucky enough to play for Real Madrid, some of us are lucky enough to form part of its history, but Real Madrid was, is and will always be Real Madrid. No one name makes the legend of Real Madrid, but we have all written that legend together.”
