You have exceeded your mailbox quota for your account. There are several steps you can take to reduce the size of your inbox for better performance:
Delete older inbox items. You can do this quickly in the app by setting a custom Swipe motion to delete: Settings > Swipe Options. With this set, you can easily swipe your way through your inbox to clear it out.
Empty your spam folders. Go directly into your spam folders and press Empty spam.
On Android, you may empty your trash folders with one click. Go directly into your trash folder and press Empty trash.
Save your attachments to OneDrive so they aren't stored in your mailbox. More information can be found here as well as other options to complete this task from a computer.
If you are a Microsoft 365 user you may also request a mailbox increase from your IT admin.