When the site collection in the read only mode, all contents in the site collection cannot be changed and deleted.
Here are steps:
1.Go to Site settings -> Site collection features -> Enable the Site Policy feature.
2.Go to Site settings -> Site Policies under Site Collection Administration -> Create a new policy.
3.Go to Site settings -> Site Closure and Deletion under Site Administration -> Choose the new created site policy as site policy -> Save.
4.Re-go to the Site Closure and Deletion -> Click Close this site now.
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