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

This error message is indicating that the linker is unable to find a symbol called "old_num_8". This symbol is being referred to from the "encoder.o" object file, but it is not defined anywhere in the code.

To resolve this error, you need to find where the symbol "old_num_8" should be defined and make sure it is properly defined. This could be in a header file, a source file, or in an external library. Once you have located the definition, make sure that it is being compiled and linked into your project properly.