Was the original database from which you copied the table split into separate front and back ends? If so, did you copy the table from the front end? If you did, this will have copied the link to the back end, which you cannot modify in table design view. The original back end file should be copied to an appropriate location on the system to which all authorised users of the database have full permissions. You should then be able to modify the new back end table in table design view. You can then use the Linked Table Manager in the Import and Link area of the External Data ribbon in the front end to change the links so that they point to the new copy of the back end.
BTW, as well as deleting the ValidationRule properties values, to allow the entry more than 35 characters into the columns in question, be sure that the FieldSize property of each column is also changed if necessary. Make it 255 to allow the maximum length in a column of Short Text data type.