Unluckily you removed the part of the error message which should provide the file and line where your error occurs. I expect it outside Polylang.
The undefined method has been deprecated since 2013. Since then, a notice was sent to inform developers that it will be remove some day. We waited for more than 7 years to remove it, but we finally did in in the v3.0 at the beginning of 2021.
So most probably you have custom code on your site, which id doing wrong by calling an internal method when a public api function does the same thing. Moreover this custom code is likely to lack maintenance for more than 7 years. Or, it’s more recent, the developer wrote it without the WP_DEBUG mode which is wrong.
]]>This is not a review; it’s a support question. I’ve moved it from the review area of the plugin to its support area.
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