To help narrow this down, could you tell us:
- Is the Microsoft account an adult account or a child account in a Microsoft Family?
- What exact message appears when you click Multiplayer?
- Is the Multiplayer button greyed out/disabled, or can you click it but not join servers?
- Are you signed into the same Microsoft account in Minecraft Launcher, Xbox app, and Microsoft Store?
The answers to those questions can help determine whether this is an account-permissions issue, a sign-in mismatch, or something else.