Hello
Thank you for your question and reaching out.
There are numerous factors that could make the Hyper-V snapshot delete option unavailable, including host connectivity issues or backup tool malfunctions. The most likely explanation is that the third-party utility that produced the checkpoint improperly destroyed it.
There's no way to delete the broken recovery checkpoints through the GUI. Instead, you must manually merge them. For information about how to merge the .avhd files into the .vhd file, see Manually merge .avhd to .vhd in Hyper-V.: https://social.technet.microsoft.com/wiki/contents/articles/6257.manually-merge-avhd-to-vhd-in-hyper-v.aspx
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