Explain the situation to the new employer. The new employer would most likely allow you to submit the relieving letter later since it is usually impractical to provide it on the joining day.
Disclosure: Rest of this answer is self plagiarised from here.
The relieving letter can be, and usually is, submitted after coming on board. It is usually impractical to submit the relieving letter on or before the joining day because:
- People usually join the new job within a few days of ending the previous job. AND
- The previous employer provides the relieving letter along with the so-called "full and final settlement", which takes a couple of weeks.
As a result, the new employer typically provides the employee up to a couple of months after joining to submit the relieving letter.