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This is not likely to be a Sage 50 Canadian problem. The problem is with the setup on your computer in possibly two different places. If you have two different log-ons for the computer you may be using one all the time that is not an administrator. While your log on will allow you to run all your programs it may not allow you to save a new file. If you are using an administrator logon then there may be restrictions on the location where you tried to save the file.
Generally when a form is modified I try to save the new form in a folder "Custom Forms" on the same drive as the backups are saved. This has two purposes: first it is on a path where the permission will probably allow the new file to be saved; and second it will not be over-written when there is an update to the Sage 50 program.