*URGENT*Clipping mask error when merging layers

Hello community!

I have to submit a ready-to-print PDF for an advertisement in a magazine. I created this with Illustrator but later on I was told that there are open layers included in the PDF I've submitted.

When trying to merge the layers, I got an error message, saying

"Parameter invalid"

so merging was not possible.

After trying out a bunch of different solutions the merging somehow worked but the clipping mask which I applied to an image disappeared and the image was left there in its full size without the mask. 

I really don't know what to do anymore ;((

can anyone please help me with this? 

Thank you!!

Helen