As you have not yet obtained approval of the I-526 petition and applied for conditional permanent residency you may still apply for a visitors visa. The visitors visa allows you to enter the U.S. for certain permissible activities. Once the I-526 petition is approved then you will be starting the process of obtaining the conditional permanent residency, therefore it will now be difficult to apply for a visitors visa which is an expression of your intent to temporarily visit the US and return to your home country, and not in the process of obtaining conditional permanent residency.
Non-immigrant, or temporary, visas like the B1 and B2 have a requirement for binding ties to one’s home country. Having a permanent residency application like an I-526 would make it very difficult to show you intend to return to your home country at the end of your authorized period of stay.