I have been using Acrobat DC Pro and up until recently was able to change a text box to transparent by right-clicking where I would find a box to check in the pop up to make box transparent; however, this box has suddenly disappeared and I cannot find out how to make the text box transparent anymore. suggestions?
By text box do you mean a text form field, or a text markup annotation?
Either way you can use the "Select Object" tool to right click on an element. Look it up on the tool search.
Are you on a Mac? Do you have High Sierra? If so, you've encountered a bug in Acrobat.