phone is not showing SD CARD Samsung A30

The card may be corrupt ie have damaged files on it or one or more files larger than 4 gig if it is formatted as fat 32. (Common). Put it in a card reader run check disk on it. In windows right click it select properties and tools and check disk for errors check the box to repair any errors found. Don't select format if your phone "sees" it and says it needs to be formatted or repaired unless you want it to possibly be unreadable in any other device... though portable storage should avoid that. If you did format it as internal storage using the phone you probably won't be able even to run check diskon it. In that event the card is now a pretty. Little tiny paperweight. But try reading it on other device's first (before you recycle it- and also try a virus scanner on it just in case...