..\OBJ\OLED.axf: Error: L6218E: Undefined symbol old_num_8 (referred from encoder.o).

This error message indicates that the linker is unable to find a definition for the symbol "old_num_8", which is being referred to in the "encoder.o" object file.

To resolve this error, you need to ensure that the definition for "old_num_8" is available to the linker. This could be done by including the source file that defines this symbol in your build, or by ensuring that the object file that contains the definition is being linked in correctly.

If you're using an external library that defines "old_num_8", make sure that the library is included in your build and that the linker is configured to link against it.