Your phone is running the Android operating system, which uses the ext4 file system. It's becoming a dated file system but still has robust limitations that are still capable enough to meet the needs of almost anyone, with the individual file limit in a directory being a theoretical 4 billion files.
A typical microSD card however is going to be formatted as FAT32 or exFAT, both being very dated file systems based on Microsoft's proprietary, no longer supported FAT. Even in its day (Win98) FAT was a weak, buggy, crash prone file system, with a now compounded problem being its stability is even less reliable with today's technology. It's strong suit being Microsoft forced the USB-IF board, the organization that determines USB protocols to adopt FAT as the default file system for USB media. So when we buy microSD cards, or USB flash sticks, or devices like webcams that have their own internal storage because of the storage media is based on USB interaction they use that outdated FAT-based file system. Basic FAT support is included in Mac, Linux, and of course Microsoft computers to things like microSD cards can be accessed and used as transfer media. But FAT32 and exFAT have file size and file number limitations based on their age.
So if you want to try and determine just what the problem is, it's really important that you acknowledge there is a significant difference between your phone's internal storage and a microSD card you might have installed. Each uses very different file systems that have very different limitations. Look in the Settings >> Storage menu to get better representation of what that error message might be trying to convey.