won't recognise keyboard

Since this mini's has been "acting up," I suggest starting with some basic troubleshooting steps and also to run an EtreCheck report to try to get things "working" again. However, I understand that if this mini is not recognizing the keyboard, the following will be problematic and it may be the only last resort is to make a Genius Bar appointment at your local Apple Store or Apple Authorized Service Provider to have them take a look.

Please try the following, in order. Stop at any step where the issue is resolved:

1. Reset your Mac's SMC.
   • Reset the System Management Controller (SMC) on your Mac- Apple Support
   • Hold down the <shift><control><option> keys, and then, press the power button. Hold all of them for, at least, 10 seconds.

   Resets fans, keyboard backlight, status indicator, battery indicator, display backlight, power button, close/open the lid, sleep or shut down, battery doesn't charge, MagSafe indicator light incorrect status.

2. Reset your Mac's NVRAM or PRAM.
   • How to reset NVRAM or PRAM on your Mac - Apple Support
   • At bootup, hold down the <option><command><P><R> keys.
   • Resets sound volume, display resolution, startup-disk selection, time zone, & kernel panic information.
3. Run either Apple Hardware Test (AHT) (pre-2013 Macs) or Apple Diagnostics (2013+ Macs) to uncover any hardware issues with your Mac.
   • How to use Apple Diagnostics on your Mac - Apple Support
   • Apple Diagnostics: Reference Codes - Apple Support
   • Use Apple Diagnostics or Apple Hardware Test- Apple Support
   • About the "Apple Hardware Test does not support this machine" message- Apple Support

   How to Use Apple Hardware Test to Diagnose Mac Problems- OSXDaily

4. Boot up your Mac in Recovery or Internet Recovery Mode to verify the startup drive using Disk Utility's First Aid option.
   • About macOS Recovery- Apple Support
   • When at the initial Recovery Mode screen, select the Disk Utility option.
   • Run Disk Utility > First Aid to verify the startup disk's integrity.

If none of the above helps, to further help troubleshoot your Mac, I would suggest that you consider using EtreCheck and posting the resultant report for us to review. (Note: The EtreCheck report does NOT contain any personal information.)

• You can download EtreCheck fromhere.
• Start EtreCheck from a normal user account. Optionally, you can run it from a user account with Administrator privileges.
• Select a problem from the drop-down menu to enable the "Start EtreCheck" button. Optionally, you can add comments on what issues your Mac is experiencing, especially to aide others with similar Mac issues.
• Click on Start EtreCheck
• Allow the program to run to completion.
• When done, select Report from the left-side window to display it.
• Select the "Share Report" icon.

  Select Copy Report

• Paste the report to your reply post.

Ref: Using EtreCheck - ASC (etresoft)

... and what have you tried, so far, to resolve this yourself? For example, did you try another keyboard, or reset the mini's SMC?