You are trying to manually edit a lead record to remove the value of the Email field but the change doesn't save.
Marketo enforces this as a required field for deduplication purposes and won't allow a direct manual edit as a preventative measure. Removing the field can create duplicates, so it's generally recommended that instead of removing the field you maintain the record in your CRM or mark this record as marketing suspended to avoid inadvertent communications. However, if you need to remove the value there are two workarounds to this issue.
1) Create a smart campaign with smart list criteria that isolates that specific lead record, with a Change Data Value flow step. Specify Email as the attribute and the New Value as NULL. This will clear out the value for that field.
2) You can remove the email address from that record in your CRM. Marketo treats your CRM as the source of authority, so removing that value in your CRM will sync down to Marketo during the next sync cycle.