The file may be corrupted and cannot be loaded into the game.

[ManHung1227]

MY version is  SIMUTRANS 121.0 r8870 .
My save has already play for a long time.
How can I solve the problem?

[Mariculous]

Use an old save, if you got one.
It is very unlikely you can fix a broken simutrans save, at least not without extensively debugging it and even in that case chanches are not good.

[ManHung1227]

i want to know what is it related about..
Capacity or Pak problem ?

[Mariculous]

*takes out his crystal ball*
Oh,  see it's definitely a program error that caused an invalid byte at position 75681 in your save file.
*puts the crystal ball away*

Well, You didn't give any details on what is happening, so it's difficult to say anything definite about it.
Though, from the title of the topic, it sounds like the savegame is corrupted, in which case there is barely any chance to recover it.
The savegame structure makes it very difficult up to impossible to detect the corrupted section.