Would you provide more information about the problem so that we can diagnose the cause of the problem?
Are you trying to sign a PDF document using the Signature tool (as part of the Commenting tools) in Acrobat DC for iOS?
Or are you using the Fill & Sign app for iPad?
It is highly likely that the problem is file-specific. Have you tried other PDF documents/forms? Does the problem occur with a particular PDF document for all of the PDF documents that you have?
Would you post a screenshot of the alert/error dialog in your next reply?