Unable to buy online tickets to Colosseum on coopculture.it site

Because they haven't opened those bookings, yet?

That is a free week for museums and sights. They used to do the first Sunday of the month, but this year they changed it. I only know this because I will be in Rome that same week and had the same issue. I’m not sure how it will work...just wait in line? If anyone else knows the logistics, I would be interested to know, too!

See this previous thread on the subject:
https://community.ricksteves.com/travel-forum/italy/free-museum-days-in-march

This doesn't mean they've done away with free first Sundays yet; they haven't announced that change that I'm aware of, anyway.