Go into terminal
Change Prompt=lts to normal
Save [CTRL]+O and exit. [CTRL]+X
# default behavior for the release upgrader
#
[DEFAULT]
# default prompting behavior, valid options:
# never - never prompt for a new distribution version
# normal - prompt if a new version of the distribution is available
# lts - prompt only if a LTS version of the distribution is available
Prompt=normal
Go into terminal
Change Prompt=lts to normal
Save [CTRL]+O and exit. [CTRL]+X
# default behavior for the release upgrader
#
[DEFAULT]
# default prompting behavior, valid options:
# never - never prompt for a new distribution version
# normal - prompt if a new version of the distribution is available
# lts - prompt only if a LTS version of the distribution is available
Prompt=normal
Try again - works on my Ubuntu 18-04.1