I tried upgrading PHP from 7.3 to 7.4 but got a message that the system detected incompatibilities. Can anyone please advise?

IPB is compatible with 7.4.  I’ve been running it for months without issue. 

My guess is when you upgraded your PHP, you missed adding a few of the required PHP modules. Run the system requirements script and it will tell you what you’re missing. 

I confirm this, I am running IPB 4.6.9 on PHP 7.4 without any problems. 😙

Edited January 7, 20223 yr by dutchsnowden

As per @Randy Calvert it will be a missing module/php function