Hi, can you try with PHP 7.2 please? Also check the tiki-check.php: server-check.
I just installed in a FreeNAS jail (FreeBSD 11.3 based) with PHP 7.4 and ended up with the same error. I did find a work around that may not be optimal but worked for me. I added the line
TIKI_PATH=/usr/local/www/tiki-22.1 (will vary depending on where you extracted tiki)
at the end of the /usr/local/etc/php.conf file. I did encounter other errors after this that I resolved to additional modules I needed to add and a few php.ini changes I needed to do. Now I have a working tiki on my home server to learn tiki. Note: after all that, I was curious and removed the TIKI_PATH definition and the site still works, so likely one of the other changes I did also resolves this but I do not know which one yet...