This information applies to all versions
This usually happens when the version of Nik Collection hasn't been detected.
First of all please make sure that your version of DxO PhotoLab and Nik Collection are compatible.
Please click this link to see the compatibility tables for operating systems and third-party software.
If you are using compatible versions we recommend reinstalling Nik Collection with admin rights. For further help refer to Nik Collection FAQ section.
macOS only:
If you can't find the button at all, go to program settings > general and make sure the option to show the button is checked.