Normally, this issue will resolve itself. If it doesn't try,
- Cancel the Download
- Go to Settings>Apps> Click 3 dots Menu on top> Show System Apps
- Search for Download Manager> Clear Cache and Clear Data.
- After a minute retry.
If that doesn't work,
5. Go to Settings> Apps> Play Store
6. Clear Cache and Data
7. Retry after a minute.
Even if that didn't work, Repeat all steps, Do a Reboot and try after sometime, May be a network issue.