The error message concerned indicates
that the MMC is no longer recognized/accepted by the SIMATIC CPU.
This is the case when the SIMATIC IDs have been overwritten, for
example through using the MMC in a non-SIMATIC application, or the
MMC has been formatted in a third-party device. Unfortunately it is
not possible to restore the internal structure of the MMC for
renewed use in SIMATIC S7-300 CPUs.
See Entry ID 21829579 for which SIMATIC devices you can use to
read from or write to S7-300 MMCs. More information on handling the
MMC is available in the manual "S7-300, CPU 31xC and CPU 31x:
Installation Instructions" in Entry ID 13008499.