Hello, Spine Essential doesn't support features that are available in Spine Professional only. Therefore, to save or export such files you will need to remove the features from the skeleton if used, they cannot be ignored.
To do so, you will need to delete all constraints, meshes, clipping, audio or other Professional feature by selecting them in the tree and deleting each instance.
Because this process is very destructive, it would be best to ask the people providing you files that use Professional features to remove them (for example they can easily uncheck meshes so that they can turn back into regular attachments), refrain from using them, or export for you.
