[Koha] Missing "merge selected" button

[Rubén Fernández Asensio]

Hi all,

For some weird reason I can't merge biblio records after searching in my 
catalog.
The manual says that after finding duplicates, I should be able of 
checking "the boxes next to the duplicates and click the ‘Merge 
selected’ button at the top of the results.":
https://koha-community.org/manual/18.05/en/html/cataloging.html#merging-records

However, what I see over the found records is only this:
"Select all | Clear all | Unhighlight | Add to list | Place hold | 
Z39.50/SRU search | Sort"

I can't see that button anywhere in the entire staff interface, even 
after selecting the records. Where should it be?

Could the reason be that the duplicate records belong to different 
branches? I've switched "IndependentBranches" to "No" but still no 
difference. And anyway, I have superlibrarian permissions.

I'm running Koha 18.05.

Ruben