Facing a error while Decryption: "The data to be decrypted exceeds the maximum for this modulus of 128 bytes."

You've mixed up string encoding/decoding and base64 conversion.

When you encrypt you store the base64 version of the encrypted byte array.

When you decrypt you're supposed to convert that base64 string to a byte array, decrypt the array and then use Encoding.ASCII.GetString to obtain the original string from the decrypted byte array. Your decryption code does exactly the opposite.

And a side note: never use ASCII unless you have specific reasons to do so. Use UTF8 or Unicode encodings instead.