I cannot click on the "OPAC" button.

Isn’t the icon of OPAC link gray? You can click only on the icon of green OPAC link.
Direct link to OPAC is set by each library, please ask the library directly about each OPAC links.

About the method of setting up the link to OPAC directly in each library, please see the following page.

About the maintenance of libraries file associated with the release of CiNii Books

We update the data of CiNii Books at the time of closing time on Saturday in predawn on following Monday. OPAC link is also in the same fashion.