We encountered an error when editing permissions for a field security profile on a newly configured field,

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However, when trying the same thing on other fields that were already enabled for field security, the changes were applied without any issues.

To uncover the cause for this error, we needed to trace back to how the original settings for field security were configured.

If the field had not been previously enabled for field security, we would have had to enable this in the classic solution explorer:

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Whether or not you receive the error after editing field permissions is determined by what steps you take next.

If you press Save without publishing, the field that you have just enabled for field security will appear in the list of fields available for field permission editing for any field security profile.

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The reason for the original error was caused by trying to make field permission changes prior to publishing the enabled field security. Once we published customizations, we were able to edit field permissions to the field in question without error.

Despite the newly configured field appearing in the list of fields available for field permission editing in a field security profile, avoid making edits without first publishing!