When clicking on register I get the following error.

Fatal error: Call to undefined method: dbal_mysql->select() in /....../......./......../......../........l......./store/includes/content/reg.inc.php on line 203

Can someone help me?

I don't see that statement in that file. Is this version CC3? Or 4?

Are there any modifications added to this store?

It's CC3.

MKportal has a mod that allows for cubecart to be integrated into the portal. After installing CC in MKportal that's the error I got.

Ah! Well, you are going to be advised to contact the author. It's truly pointless to try to troublshoot third-party add-ons in these forums sponsored by Devellion. In fact, the powers-that-be have placed such discussions out of bounds.

You will definitely have better luck going over to CubeCartForums.org. See you over there!