The main reason the user cannot add an Amendment is because this validation is looking for a CALPADS transaction with the Meeting date currently on the Student record / Future IEP with the corresponding Meeting type selected. First you as a District level user can verify if the district has a Transaction for the current IEP (which you have done), if you do, then most likely the user updated the Future IEP meeting date or meeting type triggering the validation to prompt.
To fix this:
Have the user return the Meeting date and/or type to what ever is on the Current IEP, then they will be able to add the amendment.
This validation was implemented as a safe guard to stop providers from creating amendments with the incorrect meeting date and/or type, and causing SPED0438.