If you're having trouble with a customer signing a quotation, it might be due to one of the following reasons.
Quotation Expired
If the quotation has passed its “Valid Until” date, the customer won’t be able to sign it. To fix this, go to the 
Edit button and update the Valid Until date to extend the quote’s validity.
Quotation Was Updated or Saved After Sending
If you made changes to the quotation and clicked “Save” after sending it to the customer, the quote will no longer be valid for signing. This is because the saved version doesn’t match the PDF the customer received. To fix this, simply send the customer a new version of the quote with the updated details, and they’ll be able to sign it.
Profitability Changes
If the profitability of the quote changes (e.g., prices or margins), a yellow triangle will appear next to the affected item. To fix this, click “Regulate” to update the quote with the new profitability values and send it to the customer again.
Customer Already Signed the Quotation
Once the customer signs the quote, it can’t be signed again. A message will appear to the customer stating that the quotation has already been signed.
The user withdrew the quotation from the "Pending" menu on the sales screen
By selecting the list of pending quotations and removing the most recent (active) one, you are effectively withdrawing that version of the quotation.
Error Codes
Below is a full list of error codes. These are sent to the quotation owner after a failed signature is detected to ensure the customer can be contacted to rectify the situation.
ERR010
The quotation couldn't be retrieved.
ERR011
The quotation couldn't be signed as the validity period has expired or is invalid.
ERR012
The quotation couldn't be signed as profitability has changed since it was sent.
ERR013
The quotation couldn't be signed as the signatory's network address could not be retrieved.
ERR014
The quotation couldn't be signed as the signature name was not provided.
ERR015
The quotation couldn't be signed as the signature company was not provided.
ERR016
The quotation couldn't be signed as the associated asset could not be retrieved.
ERR017/19
The quotation couldn't be signed due to an unknown reason. Please escalate to the development team.
ERR018
The quotation couldn't be signed as there is already one or more signatures for this quotation.
ERR020
A valid quotation couldn't be retrieved.
ERR021
The signee did not accept the required terms and conditions required to fulfil this order.