Hi Wendy,
Windows Defender will not function with any other antivirus, that is REGISTERED.
MalwareBytes is compatible with Windows Defender, only when it is not registered.
If Bitdefender is properly installed and registered, Windows Defender will be
disabled, by design. This is to prevent conflict.
However, if you wish to use Bitdefender as your Real-Time protection, Windows
Defender can be placed into Passive Mode, using the "Limited Periodic Scanning
switch.
In Passive Mode, Windows Defender can be used for scans only. The Real-time
protection is disabled.
To place Windows Defender into Passive Mode, you must turn "ON" the
"Limited Periodic Scanning" switch in Windows Defender. It is located on the
"Virus & Threat Protection screen. You must scroll down to see it.
The switch is only present if "another" anti-virus program is present.
Please see the following link: https://docs.microsoft.com/en-us/windows/security/threat-protection/windows-defender-antivirus/limited-periodic-scanning-windows-defender-antivirus
Having Windows Defender available for scans is especially convenient, since you
have access to the Windows Defender OFFLINE program.
Good luck, Glen