we could not complete your purchase everytime im trying to update my Macbook to Mojave. why is this happening?

hello, im trying to update my OS since a lot of apps such as office and adobe are not compatible with my current OS now. but every time im trying to update my OS, it keeps on saying that "we could not complete your purchase" and "unknown error". yes my macbook is compatible with it, because im Macbook pro 2012 and its really annoying with this error because i really need to use office and adobe again

thank you for the help