Hi,
function ToBase64String moved to 4110 "Base64 Convert" codeunit and it has following overloading methods.
ToBase64(String: Text): Text
ToBase64(String: Text; InsertLineBreaks: Boolean): Text
ToBase64(InStream: InStream): Text
ToBase64(InStream: InStream; InsertLineBreaks: Boolean): Text
Create variable for the Codeunit and pass stream as argument into ToBase64String function
for more business central breaking changes follow below link and see Base64 Convert Module
github.com/.../BREAKINGCHANGES.md
Note : make sure you are using right symbols. get latest docker container of Business Central Release Wave 2 and download symbols again with BaseApp and SystemApp Dependencies