I have it working on bootstrap 4 by using the 1.0.2 release. I suspect bootstrap 5 has some new fancy SCSS features. I tried updating scssphp to latest and got less php errors, but new ones.
Here is my log aftrer updating scss latest. if I remove the Bootstrap 5 SCSS import from the file, it compiles fine.
[25-Jul-2022 11:25:39 America/Chicago] PHP Fatal error: Uncaught Error: Class "Leafo\ScssPhp\Compiler" not found in /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/components/modxminify/assetic/src/Assetic/Filter/ScssphpFilter.php:91
Stack trace:
#0 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/components/modxminify/assetic/src/Assetic/Filter/FilterCollection.php(62): Assetic\Filter\ScssphpFilter->filterLoad(Object(Assetic\Asset\FileAsset))
#1 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/components/modxminify/assetic/src/Assetic/Asset/BaseAsset.php(94): Assetic\Filter\FilterCollection->filterLoad(Object(Assetic\Asset\FileAsset))
#2 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/components/modxminify/assetic/src/Assetic/Asset/FileAsset.php(65): Assetic\Asset\BaseAsset->doLoad('@import "bootst...', NULL)
#3 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/components/modxminify/assetic/src/Assetic/Asset/BaseAsset.php(103): Assetic\Asset\FileAsset->load()
#4 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/components/modxminify/assetic/src/Assetic/Asset/AssetCollection.php(151): Assetic\Asset\BaseAsset->dump(NULL)
#5 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/components/modxminify/assetic/src/Assetic/AssetWriter.php(68): Assetic\Asset\AssetCollection->dump()
#6 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/components/modxminify/assetic/src/Assetic/AssetWriter.php(53): Assetic\AssetWriter->writeAsset(Object(Assetic\Asset\AssetCollection))
#7 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/components/modxminify/model/modxminify/modxminify.class.php(187): Assetic\AssetWriter->writeManagerAssets(Object(Assetic\AssetManager))
#8 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/cache/includes/elements/modx/revolution/modsnippet/22.include.cache.php(13): modxMinify->minifyFiles(1)
#9 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/src/Revolution/modScript.php(88): include('/mnt/target03/3...')
#10 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/src/Revolution/modParser.php(505): MODX\Revolution\modScript->process(NULL)
#11 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/components/pdotools/src/Parsing/Parser.php(273): MODX\Revolution\modParser->processTag(Array, true)
#12 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/src/Revolution/modParser.php(218): ModxPro\PdoTools\Parsing\Parser->processTag(Array, true)
#13 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/components/pdotools/src/Parsing/Parser.php(70): MODX\Revolution\modParser->processElementTags('', '<!DOCTYPE html>...', true, false, '[[', ']]', Array, 9)
#14 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/src/Revolution/modResource.php(521): ModxPro\PdoTools\Parsing\Parser->processElementTags('', '<!DOCTYPE html>...', true, false, '[[', ']]', Array, 10)
#15 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/src/Revolution/modResource.php(469): MODX\Revolution\modResource->parseContent()
#16 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/src/Revolution/modResponse.php(72): MODX\Revolution\modResource->prepare()
#17 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/src/Revolution/modRequest.php(154): MODX\Revolution\modResponse->outputContent(Array)
#18 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/src/Revolution/modRequest.php(138): MODX\Revolution\modRequest->prepareResponse()
#19 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/src/Revolution/modX.php(1495): MODX\Revolution\modRequest->handleRequest()
#20 /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/index.php(63): MODX\Revolution\modX->handleRequest()
#21 {main}
thrown in /mnt/target03/358650/modx3fred.modxwireframe.com/web/content/core/components/modxminify/assetic/src/Assetic/Filter/ScssphpFilter.php on line 91