Hello Rudá
As you are using Moodle 2.6.1+, you ned to go to this page:
https://github.com/jmvedrine/moodle-block_verify_certificate/tree/MOODLE_26_STABLE
Click on the "Download ZIP" button on the right.
You will get a zip archive with a name like moodle-block_verify_certificate-MOODLE_26_STABLE.zip
Extract and rename the folder from "moodle-block_verify_certificate-MOODLE_26_STABLE" to just "verify_certificate" (verify that inside this folder you see db and lang subfolders and 5 files)
copy this "verify_certificate" folder in the blocks subfolder of your Moodle server (if there is already a verify_certificate folder, delete it before copying the new one) Connect as administrator and install as usual going to Site Administration Notifications.
Report any problem here.