How do I fix this error:
Hint: You can set the maximum SRAM memory of your board directly in the boards.txt file
It is not an error, it is just a hint, letting you know that if you added the maximum sram to the boards.txt entry for that board, the IDE could display what % of SRAM is used by globals, like how it displays available flash. You can safely disregard it if you choose with no ill effects.