Hi G man,
The card is likely formatted incorrectly, you will need to reformat it to FAT or ExFAT ([link|https://www.wikihow.com/Format-an-SD-Card|guide]), also, this phone only [link|https://www.samsung.com/uk/support/mobile-devices/what-size-memory-card-does-my-smartphone-accept/|supports cards up to 400GB]
[link|https://downloadcenter.samsung.com/content/UM/202001/20200129151215550/SM-G96X_UM_EU_QQ_Eng_Rev.1.0_191210.pdf|Samsung user guide:]
[quote|format=featured]Memory card (microSD card)
Installing a memory card
Your device’s memory card capacity may vary from other models and some memory cards
may not be compatible with your device depending on the memory card manufacturer and
type. To view your device’s maximum memory card capacity, refer to the Samsung website.
• Some memory cards may not be fully compatible with the device. Using an
incompatible card may damage the device or the memory card, or corrupt the data
stored in it.
• Use caution to insert the memory card right-side up.
• The device supports the FAT and the exFAT file systems for memory cards. When
inserting a card formatted in a different file system, the device will ask to reformat
the card or will not recognise the card. To use the memory card, you must format it.
If your device cannot format or recognise the memory card, contact the memory
card manufacturer or a Samsung Service Centre.
• Frequent writing and erasing of data shortens the lifespan of memory cards.
• When inserting a memory card into the device, the memory card’s file directory
appears in the My Files → SD card folder.[/quote]