@Md Talukder Are you on the Free plan? If not, which plan are you on?
For other readers: the Free plan has historically allowed unlimited boards, but only the three most recently created are editable — older ones lock to view-only. The usual scenario:
1. You're on the Free plan with three boards, all editable.
2. You create a fourth board — one of the original three automatically flips to view-only.
3. To edit that view-only board without deleting anything, you duplicate it — the duplicate takes an editable slot.
From this post from last week it appears that Miro has applied further restrictions to the free plan to only allow three boards in total. I tested this change myself in a free plan that I have and I was not able to create more than three boards. However, I didn't try to duplicate a board.