Under your Email & Phone settings, we have a 'Sending Safety' feature that can be used, where you can set a threshold to ensure a record doesn't receive more than 1 email in X amount of hours or days. (see the below screenshot)
If you have the sending safety feature turned on for Sequences, if the sequence steps are set as such that an email is supposed to go out, and it is within the settings you ahve set, the contact will move into the Sequence issues tab until the time has passed, and then the email will be sent.
If you have this turned on for manual emails (uncommon), then you will receive an error that will state "could not send email due to email safety restrictions." In this case, it's likely you sent an email and then replied, and you are trying to reply within a day. To fix this, simply toggle off the setting to have this applied to Manual Emails.