❔ Base64 invalid string error while the input is valid

so im working with base64 that has to be decoded, i made a handy method for it so i can do it easily whenever i need. but it says the base64 string was invalid. i went to an online decoder and it decoded it to exactly what i needed it to be. so now im really confused :/
perhaps someone here could lend me a hand

context provided in the attached screenshots