Apps from Mac App Store not installing

First post on here please bear with me.

I bought the utility by MacPaw - Gemini. I completed the purchase and I could see the app icon in Launchpad with the white progress bar underneath it.

Just before the progress bar completes, the icon disappears. I come out of the Launchpad screen back to my desktop to see the following error message:




I tried download another free app (Caffeine) and the same problem occurs.

When I look back in the App Store, the two apps show up under purchases, but they have the Install button next to them. When I click this I see the progress bar appear for a split second and then the same error pop up as before.

Could anyone please help with this?

Thanks

I have also just tried to update an app, and have got the same error message as per my original post.

Any help greatly appreciated as it now appears that my Mac App Store is really screwed.

Without a fix my updates will just stack up.

I am a bit out of my depth on this one.