Bluetooth is not working ubuntu 18.04 LTS

I had the same problem and found this:

"A brilliant developer named Takashi Iwai came up with a solution that I incorporated into the 4.15 kernel

sudo apt install git build-essential dkms
git clone https://github.com/jeremyb31/newbtfix-4.15.git
sudo dkms add ./newbtfix-4.15
sudo dkms install btusb/4.0

Reboot"

Found here ubuntu 18.04 LTS bluetooth discovery not working

I had to install git manually from Synaptic. I don't understand what it does, but it worked for me.

I partially fixed the problem running the following commands:

sudo apt-get install build-essential git
git clone https://github.com/lwfinger/rtlwifi_new/
cd rtlwifi_new
make
sudo make install
echo "options rtl8723be fwlps=0" | sudo tee /etc/modprobe.d/rtl8723be.conf

you might face a problem of a slow wifi connection after that, I fixed it with following commands:

sudo modprobe -r iwlwifi sudo modprobe iwlwifi 11n_disable=1
echo "options iwlwifi 11n_disable=1" | sudo tee /etc/modprobe.d/iwlwifi.conf

IMPORTANT: This did not fix the problem totally, I have to close wifi, search and pair bluetooth device then open wifi again. I don't know exactly why is this happening but googling told me that ubuntu drivers are problematic with rtl8723b adapters like mine

I had the same problem after upgrading the system. I found following error in /var/log/dpkg.log :

2019-03-23 16:37:24 status half-configured pulseaudio-module-bluetooth:amd64 1:11.1-1ubuntu7.1
2019-03-23 16:37:24 status half-installed pulseaudio-module-bluetooth:amd64 1:11.1-1ubuntu7.1
2019-03-23 16:37:24 status config-files pulseaudio-module-bluetooth:amd64 1:11.1-1ubuntu7.1
2019-03-23 16:37:24 status config-files pulseaudio-module-bluetooth:amd64 1:11.1-1ubuntu7.1
2019-03-23 16:37:24 status config-files pulseaudio-module-bluetooth:amd64 1:11.1-1ubuntu7.1
2019-03-23 16:37:24 status not-installed pulseaudio-module-bluetooth:amd64 <none>

So I reinstalled the package, and restarted pulseaudio, which finally solved the problem:

sudo apt-get install --reinstall pulseaudio-module-bluetooth
pulseaudio -k
sudo alsa force-reload

I'm on Mint and started getting this same "last event is not cmd complete (0x0f)" bluetooth network connection problem after the kernel updated to 4.15.0.33, nothing I did would fix it, but I booted into 4.15.0.32 and it worked again.

So since Ubuntu 18.04 also uses the 4.15 kernel, it's quite likely that you're experiencing the same problem, try booting in on 4.15.0.32 or less and see if that fixes the problem.