Welcome to Android Central! If you still want to use the card as Internal Storage, first you have to make sure the phone supports Adoptable Storage. When you try to format the card in the phone, do you get prompted to select Portable vs Internal Storage? If not, then the phone doesn't support Adoptable Storage.
If it does, then be aware that the card will be wiped in the process, so you'd have to back up all of the data on the card to somewhere else before you re-format the card as Internal Storage. Also be aware that once you format the card as Internal Storage, the card is encrypted to that phone -- you can't take the card out and put it into another device to read. And if you were to factory reset the phone at some point, that "instance" of the phone would be gone, and the encryption key with it, meaning that all of the data on the card would become useless.