What can I do if my MacBook Pro keeps shutting down in sleep mode?

It is possible it may be due to Kernel Panics as well. I know many SSD failures are found when waking the computer from sleep. Are there any Kernel Panic logs located in "/Library/Logs/DiagnosticReports" or the "Retired" subfolder? Kernel Panic logs usually have "kernel" and/or "panic" in their file names. If you have any Kernel Panic logs, then post several of them here using the "Additional Text" icon which looks like a piece of paper on the forum editing window toolbar.

sallyfrommundelein wrote:

I have an old MacBook Pro that I hAVE just started using again after many years. it keeps turning off when it. should go into sleep mode.

[Re-Titled by Moderator]

MacBook Pro 15″, macOS 10.15

Catalina 10.15.7.....(?) ref: Earlier Operating Systems - Apple Community

I do not have access to your earlier macOS...

However I would verify your Settings—

ex Sequoia—

sallyfrommundelein wrote:

I have an old MacBook Pro that I hAVE just started using again after many years. it keeps turning off when it. should go into sleep mode.

is this plugged into power...(?) I would keep it plugged in.

For power management you can try resetting the System Management Controller Reset the SMC of your Mac - Apple Support for your issue.