expl3.sty not found, when using siunitx

the problem arises when people change the position of files within the CTAN repository so the local data base on your PC has the wrong address to download the necessary packages. In order to solve this issue follow those steps:

1) start: miktex -> maintenance -> package manager(admin) //you need to be admin to install this



2) Then you need to update the data base of the package manager:
in the package manager:  Repository -> synchronise




3) with in the package manager GUI install the necessary packages, in this case:
- l3kernel
- l3packages
- siunitx






4) In order to apply the changes you need to refresh the local data base
start: miktex -> maintenance -> settings (admin)
click on:  - refresh FNDB
                - update format